my engl savage se sounds far superior to any solid state i've tried at low volumes (unless you're trying to run it at like TV volume, where it can sound a bit muffled/flat/compressed/fizzy).
it does depend on the amp as well, and what style of amp it is (if it has no master volume, or a rubbish master volume), what tones you're after, etc., etc. as rarely are any situations alike.
I think the argument that tube amps sound rubbish at low volumes is total cr@p. They definitely (again, depending on the amp) sound better at higher volumes, no doubt about it, but i certainly wouldn't say they sound cr@p at low volumes.
As ben says, you could consider an attenuator if it really worries you.