I've never agreed with the idea that taking something plus or minus 3 dB in a mix (live or recorded) is "barely noticeable". Try that with any track in a halfway decent mix in your DAW and you will notice a substantial difference. Even going +/- 1 dB will yield a clearly discernible difference. A few dB, live, can make the difference between feeling relaxed or dynamically frustrated through the night. If you have a bad mix (problems other than relative levels) or other, acoustic difficulties, though, turning up often won't help (and you still might not be able to hear yourself or be heard clearly in the audience). --- As for running one or two amps with two cabs yielding more volume or not (re. the OP), I think that's been covered pretty well -- depends on where the bottleneck is. Some amps feel different running at different loads (or running closer to their output limits), so that might be a factor for some.