This thread is made of win and own.
Yes, there are many valve amps that sound great at low volumes and there are many ways to make them sound better at low volumes that are simple and easy (attenuators and ODs, for example).
The biggest limiting factor I find isnt the valves, its speakers. I dont think that you have to play at just 'loud TV' levels to get speaker excursion. Its usually a lot louder than that in my experience. It depends of the speaker, obviously, but it takes a bit to get them moving to get real purcussion and swing in your tone. But them, how much you need that really depends on your playing style too.
Obviously the limitation of speaker excursion is somthing SS suffers from too.
P.S. - Currently using a powerball through an uberkab, and it sounds great at levels you can talk over. Stuh nailed it: somewhere down the line the fact that valve amps sound their best at high volume mutated into they only sound good at high volume, and its just not true.