I spent a lot of time (and money) trying to get a 'marshall stack' tone at apartment levels. I had John McIntyre make me several low power amps, starting with a stereo 5 watt, and then 2 versions of stereo 3 watts (the prototype became the Lexicon Signature 284).
One thing that makes an incredible difference that I haven't seen mentioned here is the efficiency of the speaker that is being used. Going from the least efficient but commonly used 12" to the most efficient (and still common), there is a difference of almost 10dB. This is like going from a 5 watt amp to a 50 watt. If you want to drive a low power amp to get a good sound at lower volumes, get the least efficient speaker you can find. The caviat to this is that you do need to have some power to drive it! My Minimat sounds prettygood through the 20watt G12M heritage, which has only a 93dB efficiency, but using a Blue Tick Hound (99db efficiency), gives it more guts.
There is also the interaction of the guitar with the speaker, which only happens with sufficient volume to create a feedback loop (unless you are using a pedal).
I have started to think that it is cheaper to buy everyone else earplugs that to get a good low volume tone!!