I have a 50 watt 1x12 Hiwatt combo currently loaded with a 75 watt/ 16 ohm Celestion Creamback. To my ears, this speaker is very "scooped"; emphasis on lows and highs and not a lot of mids. I like the speaker. It seems to have a good deal of headroom (I play loud and clean and don't look for distortion unless a stomp box is involved).
I have a 1x12 extension cab I want to run but, I don't want to load in another Celestion. I would like the replacement to have a good deal of headroom and have a bit more high end clarity and punch. Two Warehouse models come to mind: the G12C/S and the Retro 30.
That's difficult to say with certainty as we couldn't say what kind of highend you're looking for. Though obviously a 50oz speaker is bound to have more highs and lows than a 35oz one would, but also more headroom.
Does the Hiwatt have an extension out for starters, btw?
I would suggest the Retro 30 or the ET90 if you want to go with WGS, heavy magnet and prominent mids. Perhaps even the Liberator 80 or Classic Lead. The G12C/S is designed to add colouration to your tone and I could see it doing that extensively with a Hiwatt.
I ended up going with the Retro 30. This was also recommended with a phone call to WGS. The end result was exactly what I was after; more punch in the high end and a bit more mids. Cranked up, the rig has plenty of headroom using single coils in the 5.6k to 6.3k range. I haven't run humbuckers through it yet.
To answer your question, the Hiwatt has two speaker jacks wired in parallel; one for the internal speaker and one for an extension.