Here’s my take. First, you’ll notice the sensitivity is virtually the same for the ETxx’s, Vet 30, HM75, Retro 30 and Reaper HP – a good sign, but not a guarantee of a good mix in and of itself. Second, the Fs for each speaker is fairly close, so the low to high end response should remain essentially balanced – also good.
The cab Rs depends on the internal volume to speaker surface area, so a 2x12"cab configured for the same Rs as a 4x12" cab should have the same output up close across the spectrum when powered by the same wattage, but with more mid-bass and higher at 2x off-axis, and progressively less mid-bass further away on-axis as is it projected more forward from a wider speaker surface. A taller speaker array with the same power wouldn’t affect the horizontal mid-bass dispersion at all. Of course, two 2x12" cabs with the same Rs placed together will have the same bass anywhere as one 4x12" cab with the same Rs, but some internal midrange peaks and valley’s will be different due to the internal dimensions – can’t say without extensive calculations. Placing the cabs far apart would of course create a progressively lower frequency cancellation as you move toward one cab from the middle. Should actually be less cancellation closer to either speaker and none right in the middle, but room reflections can affect that.
A 6x12" cab with all the same speakers would be almost 2dB louder than a 4x12" cab up to ~800Hz to 45d off-axis. The range above that depends on how close to on-axis you are. There may even be an upper-mid dip somewhere between ~45d and ~20d off. A vertically stacked 2 0r 3x12" array essentially doubles the horizontal dispersion angle at the same frequency. Two of those cabs would of course allow a much more even stage level up to the 4kHz+ presence range, so everyone can hear what you hear and no one gets blasted with the strong beaming affect of a 4x12". Upper-mid beaming comes from the speaker edges and would take a large baffle structure to disperse. I think ~20d is a good slant angle to evenly disperse 4kHz ~45d off-axis for most 12" guitar speakers. The top speakers could also slant vertically ~20d for optimal dispersion from a single cab, but that gets more complicated to build. Use thick unshielded cable for the speaker wire so there isn’t any high end loss in the cab across the stage. Speaker wire does not require shielding. It actually traps heat – not good.
Rather than bother with the expense and hassle of custom cabs, you might get get two ~$300 Jet City ‘24SVe’ 2x12" slant-offset cabs and replace the upper-mounted EMI V75 with an ETxx, HM75 or Reaper HP for more presence. Those cabs are a great design, and a real bargain with the speakers included. The extra width to cone surface area should give it a lower than Marshall cab Rs for a deep warm punch. It should disperse 4kHz t0 ~20d on the V & H axis, so two of them would offer lots of dispersion options, and be much easier to carry than 2 x 3x12 cabs – one under each arm for a strapping young roady. I imagine the Jet City EMI V75 speaker is a ‘Legend 1275’ – close enough to the Vet 30 that you certainly wouldn’t notice the difference mounted on the bottom. Virtually all the ~4kHz+ you’ll hear standing up from at least ~5’ away would come from the top speaker, with up to ~2kHz coming from the bottom EMI speaker for a tad more mid “Metal” than by itself.
You can play the WGS Mesa Recto clips of two different speakers at the same time to hear how they blend (assuming the recorded levels are the same). I captured SPL graphs of the Mesa Recto clips that show the differences in the high end. They look pretty accurate compared to official SPL graphs of similar speakers, but for a few minor cab-colourations. The more harmonics added to the guitar sound, the more filled in is the high end to better represent the speaker spectrum. They must have been close mic’d on-axis with a pretty flat mic. The low end is too cab-colored to trust, so I’d ignore that. Lot of speculation to follow, but it all looks and sounds to add up.
Although brighter than the ET65, the added low end up to ~2kHz from the bottom EMI V75 should actually make the ET90 sound more like the ET65. That and the fact that the higher power “motor” should better match the V75 bass output as well as smooth out the high end peaks for a less ice-picky ~3.3kHz with a tad more ~4kHz chime than the ET65. That’s what more motor power generally does, and what the ET90 sounds like to me. AAMOF, the few dB more low and high end should effectively reduce the ice-pickiness even more, while the strong 2.2kHz Metal from the bottom V75 will add some nice definition. If the ET90 does indeed have more midrange as well as more low and high end, it may be from less mid-dip due to the tighter suspension or something – hardly noticeable when combined with another speaker. All those factors should make it the better choice for that cab.
My SPL graphs show the HM75 to have virtually the same presence range level and shape as an ET65 with a few dB more ~2.4kHz metal and a rounder 3.3kHz peak with more mid-scoop like the EMI V75. The lower Fs and stronger motor also gives it a slightly deeper/stronger bass punch. The Retro 30 clip seems to dominate the entire midrange in the mix with a more nasty/crackly/messy upper-mid character that doesn’t sound very “post rock” at all. It could be smoothed out with a different amp configuration, so I won’t dismiss it entirely. The Reaper HP sounds really good with the Retro 30, ET65, HM75 or Vet 30. It’s full, balanced and tight with a nice balance of metal to woodiness the ETxx’s and HM75 just don’t have – I assume the wide felt dust cap damps the upper-mid Clarinet-like resonance. You might consider it as the default speaker to mix with. I know from other sources that it’s superb combined with the Vet30. My choices to mix with the EMI V75 in order of preference and for your needs are: Reaper HP, HM75 or ET90 (depending on your presence and midrange preference), Retro 30 (only with a cleaner/smoother amp section sound). I would not use different speakers in the top of each cab, but the options I suggested are are similar enough, with the possible exception of less 4~6kHz in the HM75, that it won’t really matter.
Along with the new speakers, I very much recommend trying a set of JJ KT77 and/or 6CA7 power tubes and also trying both a JJ ECC803S long plate and a ECC83MG medium plate other than a standard short plate ECC83S in the PI position of one of your amps. Most of what we may think is power tube overdrive when the amp is cranked is actually PI tube overdrive, so the break up character and output level of the PI tube in relation to the power tubes, OT and speaker is essential to the character of the amp. Most players judge tubes without really understanding how the PI tube to speaker path affects the sound. I’d configure it first, and then choose preamp section tubes to fit with it. The lower-gain/higher-output JJ ECC803S should lower the mid-emphasis as the power tubes are driven harder with less PI tube hash and more power tube sparkle compared to a short plate ECC83S at the same Master Volume setting. The lower-output/high-gain ECC83S raises the power tube push to the upper-midrange and also generates more PI tube hash, while the ECC83MG is in between those two. So, you can adjust the amp overdrive character/feel with combinations of PI and power tubes along with the tonal resonance qualities of your speakers. I’d say it’s worth trying compared to the cost of your cab set up.
The KT77 is a drop in EL34 replacement that will give an amp deeper tighter lows and a less messy extended high end compared to an EL34. The JJ KT77 “beam tetrodes” are excellent NuMetal tubes for a rich clear harmonic sound and a less compressed midrange than an EL34 without the mid bloat and low end flabbiness of an overdriven 6L6GC – holding together better under stress. I agree with R. Bogner that “the KT77 is what an EL34 wants to be when it grows up.” A long or medium plate PI tube might be better with those, but you’ll need to experiment with how the midrange and high end sounds with the amp section at different drive levels. The JJ 6CA7 is a sort of beam tetrode EL34 modeled after the original GE 6CA7. They sound like a tighter GE 6L6 with the beefy lows and sweet high end shimmer, or a lot like a JJ KT88 with a more-compressed EL34-like midrange – all may be due to the higher than 6L6GC current draw. Darker sounding than the KT77, you might prefer a short plate 83S or 83SMG PI tube with those. The more I think about it, recalling the sound of my 70’s era HiWatt combo with the genuine GE 6CA7 tubes, the more I think you might prefer them over the KT77 – tough call. The JJ EL34L is a modern sounding EL34 with a more aggressive midrange, cleaner extended highs and uber tight lows. They may sound too mid-aggressive, and maybe more mid bass “murky” for some reason, and with more chaotic swirly upper-mids when driven hard. I’d assume a short plate PI tube best with those. You may already have them in either Marshall – highly recommended for the DSL. The EL34II is a type of high gain EL34. Rather resonant & raw – more hard-rockish I’d think. Here’s a good comparison of the JJ EL34, EL34L and KT77: https://www.youtube.com/watch?v=xjgKtpTjK0o
Not much out there fro the JJ 6CA7 but this awesome camera mic’d '71 100W Marshall SL demo: https://www.youtube.com/watch?v=0g11AUYukfQ
They all sound good in their own way. The crisp high end of the Reaper HP may benefit from the sweeter upper-harmonic clarity of a beam type JJ KT77 or 6CA7 power tube, although the EL34L high end might be sweet enough with a short plate PI.
I recommend JJ tubes because they are considered the most robust modern tubes and tend to hold together better when overdriven without getting mushy, harsh or grainy. I also recommend buying them from Eurotubes.com. They sell JJ exclusively and at good prices with the most thorough testing and understanding of any tube testing house. You should also try some of their Tube Damping Rings for at least the ECC803S and really for any power tubes subject to strong vibrations. They are inexpensive and made of a semi-soft, dense, high temp Silicon perfect for the job. Tubes can still vibrate in a head atop a cab from bass vibrations which disperse everywhere – vibration is the valve killer. You might get the Hot Tube Grabbers as well, and the ET “Pro One” Bias Probe looks like the best such device for easy/safe/accurate bias adjustment while the amp section is running. Again, a worthwhile investment compared to the speaker cab costs. Oh, and always run tubes vertically to avoid potential filament rattles or shorting.
OK, I could go on with more set up recommendations, but I think this is more than enough for now. Let me know any thoughts and questions.
