It is about volume. And that is what sound pressure is. The twin has two speakers, truly, while my other amp has one (Fender Twin Reverb Custom 15") Summing the individual pressures created by the speakers yields the total perceived loudness and, of course, total speaker surface area contributes to the eventual pressure created. There are many factors that contribute to an amp's perceived loudness, to be sure; speaker sensitivities, running the power section while clipping in a tube rig (versus avoiding clipping in a SS amp), input gain, preamp gain(s.) I supposed I should rephrase the original problem.
When I play my guitar using an "empty" patch, I can achieve a volume that is loud, but not acceptably so for live performance in the situation I mentioned. When I call up an amp model, I can increase the gain of various parts of the tone stack to increase loudness. When I add, say "EQ+Gain" pedal, I can (not surprisingly) increase gain and, therefore, loudness. When I add gain to various EQ channels, I further increase loudness. Etc., etc. I am looking for a hint or two to get from the empty patch volume to something approaching a Twin. My guitar is one of a few Jazzmasters. My only outboard effect is the Fender 6G15 Reverb tank, wired into the FX loop (before the amp sim in the signal chain.) My models of choice involve only clean amp sounds (Fender Blackface models, etc.) My gut tells me that if I was using a high-gain model like a Soldano or something, then I would have far less trouble getting a prodigious stage volume. However, I have no use for those grainy, distorted models. My confusion is compounded by the myriad ways that you can tweak the volume with this amp. When I am dialing in a sound I have seen lots of possibilities: amp master volume, amp drive, tone stack volume (via gain bass/mid/treble gain), overall amp volume (via gain), pedal volume (via gain), FX loop volume (via send/return gain), compressor volume (via gain), EQ volume (via the various gains), and so on... Maybe I should peg a lot of these and see what happens? In my chain, compressor, EQ, and FX loop are post-amp so I feel I could safely increase gains here and not get any modeled preamp distortion.