Honestly, the whole idea of a good power amp is that it isn't supposed to color your tone much so that your preamp stages shine. If you subscribe to that theory, you shouldn't be all that concerned with your power amp (assuming it's high quality) because the bulk of your tone is in that Quad.. You should just choose the wattage you need. however, if you want to get some coloration from the power amp tubes you are going to need to really drive them. That equals volume! I've got a 295 that I've never been able to get much power amp distortion out of simply because even at 30x2 it's really, really **** loud.
But the truth of the matter is, you aren't going to (and probably shouldn't) get a whole lot of coloration from a 20 watt or more power amp at bedroom levels.
That isn't to say they all sound the same, the tubes all have a little character to them, but honestly it isn't going to impact your sound a ton at those levels.
Personally, given the bands you mentioned, I think the "classic rock" sound you are looking for is probably 12AX7s into EL34s.
Because of that, I recommend you audition a Peavey 50/50, although the Mesa 20/20 has a similar sound with lower output due to the EL84s. It's also has a bit more midrange and breaks up earlier - could be important given that you don't want high volume. But the Peavey 50/50 goes for half the price of the Mesa 20/20, and you could put some Yellow Jackets in with EL84s and take it down to 20-30 watt level.
The Mesa 50/50 is 6L6 based, and has a different feel, not to mention you'll be into some serious volume by the time you get them to break up for you. It's a great amp that I love, and if you can give one a listen, but I'm not sure it's what you are after. You could use Yellow Jackets to swap the 6L6's for EL84s, but you might as well just have gotten a 20/20 then.
The 2:90 series, even despite being extremely loud by the time they break up, are also 6L6 based, but you can swap in some EL34s and bring them down a bit. They'll still blow you away though, even on half volume.
The 295 can be switched into class A where it runs just the EL34s, but it's ear blisteringly loud even at just the 30 watts as I mentioned. It also stays super clean as long as possible.
I can't comment on the 395 or 500, I've never heard them in person.
So long story short: If it has to be a Mesa, get the 20/20. If you are open to other ideas, try a Peavey 50/50, or otherwise slave some EL34/EL84 based heads in the 5-15 watt range. My feeling, though, is at low volume levels you aren't going to hear much of a difference between power amps.. you might even save yourself some money by experimenting with some SS stuff.