I’m only going to address your question, vs opinions on the guitarist volume.I run an Aguilar tone hammer 700 and a single 112 Aguilar 8ohm cab.
This band that I just joined practices at such a high volume but I can't hear the detail in my playing that I would prefer.
The Aguilar cab is rear ported.
I'm considering a new 4ohm cab so that I can get the full 700 Watts out of the amp.
I do own a second 112 8ohm cab but I hate to have to schlep two cabs and an amp to every rehearsal.
Will a single 4 ohm cab make a big difference? Would a front port vs rear Port make a big difference.
Suggestions welcome.
We don’t play that loud for rehearsals, but I use a TH500 and either SL112 or 2x8” or 2x10” all 8ohms.
For gigging I was using my TH700 with SL212 at 4ohms which is plenty loud and can keep up in the environment you describing… but was too heavy to lug around so I picked up a GR AT212 4ohms at 28lbs it’s great and very articulate. The Barefaced Super Twin 2x12 would also be a super loud cab that would let your TH700 make a statement as well.
I’d definitely wear ear plugs in your situation as severe volumes are not necessary for small venues and rehearsals.

This is what I did, and it works, send about 1450 watts into two 18's and two 15's loaded with JBL's, run the 18's clean and the 15's obnoxious, it will sound great, and your guitarist's looks on their faces will be permanently altered.