in my experience, a 10 watt tube amp will USUALLY spank a 10 watt SS amp, but that's not because the watts are different-rather, it's because the people designing the vintage tube amps that comprise most of the tube amps i've played (the others generally being retro-styled tube amps that probably use similar design ideas) were more "honest" with their specs than the SS guys, who certainly sometimes mix up peak and RMS if nothing else. in the end, a quality, efficient SS amp will be every bit as loud as a similar tube amp.
all that said, i have a little SS practice amp (one of the peavey rage series) that is absurdly loud. at 5 (it doesn't get much louder than at 5, just breaks up more), it is AT LEAST as loud as my '70ish fender musicmaster (which also doesn't get louder past 4 or 5), my crate vc20 (that thing has monster bass, so i'm sure it gets boomier above 5), the marshall ministack (dual 12 cabs) i comped it to (gets louder above 5), AND my PRS half-stack (70w head, gets plenty louder than it is at 5, but at 5 it seems no louder than that damned rage at 5). it also far and away destroys any other rage i've ever heard, except for the same generation that i've heard it against (i used to play in a band with 3 other people that had them-i think the first generation of rage amps were just really loud). in any case, your milage may vary. in the end, a GOOD amp is usually louder than a mediocre amp, not because loud is good per se, but because well-designed amps have both honest power specs and efficient, clean circuitry that allow quality, complete signals to come out.
