A knowledable Gent I once worked for explained wattage and db levels to me like this... 'It takes 10 x's the wattage to double the decible level' From this it reasons a 100 watt amp will only be twice as loud as a 10 watt amp, but the clean headroom it gives you is incredible!

There are many amps out there that operate at half wattage like the Marshall Jubilee 2550, or the Traynor YCS50 which can run in a 15 watt mode.

I don't think either of these amps run at the lower wattage really reduce the overall volume much, but it does give them a different feel.