New To changing out speakers, need advice.

As the title states I’m new to changing out speakers to change my tone. I’ve read a lot here and on a few other forums and think I have an idea of what I want, but I want to get some advice and see if if matches up with my thinking. I’ll start with my gear…

JCM 900 4100 with 5881’s
JCM 900 2100 with El-34’s
(I have serious plans on a Bogner 101b soonish)
JCM 900 1960a 4x12 cabinet with stock g12t-75’s in it

American made PRS Tremonti signature
Gibson LP custom (stock pup’s)
1987 Fender Strat contemporary

Right now my priority is to get these 900’s sounding closer to what I want, and if it goes well with the bogner too then cool but I’ll probably get another cab for it.

The sound I’m trying to achieve would be 80’s Metal I guess. Vivian Campbell on the Last in Line album is the best reference I can think of to what I want. Right now I’m getting a lot of shrill and nasally tones out of my sound that I can not stand. Totally puts me out of the mood to play when I hear it. I’ve come to understand those 12t-75’s are notorious for sounding like that. My setup actually records well but in a band setting or in the same room as it, it drives me nuts. I want to replace all four. I am open to doing a mix of two different speaker but at not opposed to having all 4 the same either. Thanks in advance.

Last in Line was recorder in late 1983 at Caribou Ranch then mixed and released in 1984. Caribou had TONS of gear … so who knows what he played through!
But to me it sounds very much like the classic greenback tone … so that would be a quad of Green Berets.
Since you say “Right now I’m getting a lot of shrill and nasally tones out of my sound that I can not stand” … The other good option would be a quad of ET65’s … which would sound even more organic and less shrill than the GB’s!