Originally Posted by Peteus
Depends on the amp.
More speakers, like a 4x12, is technically always going to be LOUDER since more = more, but there's some difference between 'perceived volume' and 'actual volume'. If you're worried about your neighbors, yeah, your amp on '5' with a 4x12 will make more noticeable sound in adjoining spaces than a 1x12 and your amp on the same setting.
The main issue is that 1.) tube amps sound like shit turned down; no cabinet, big or small, will fix that 2.) speakers always sound good when they're 'pushing air', which can only happen if there's enough space for it to breathe and the volume turned up enough to make the speaker move. By both measures, "quiet" for a tube amp is all relative to what you can get away with where you are. Solid state or a modeler will probably be more forgiving, but you speaker still want to be MOVING to sound any good. That's all going to be dependent on how restrictive your living situation is.
Personally, if I wanted the best of both worlds, I'd have either a ported oversized 1x12 (like the Thiele or other ports I mentioned) or an oversized closed-back 2x12. Both are designed to move a little more air than a closed 1x12, will have less perceived volume than an open 1x12, and will produce less actual volume than a 4x12.