What kind of speaker footprint do you want? Also, what kind of "sound" are you looking for, if any? Your speaker choice/capability will determine what is suitable as an amp. For example, a pair of Klipschorns would run fine on a Dynaco ST-70, but my Infinities would go into protection if I tried to use one - even though the speakers are of similar caliber.
Of course, as others have mentioned your room will dictate what is suitable for speakers too. As long as you have a decently sized living/listening room for them, I am honestly a fan of floorstanders over monitors on stands. They don't really take up a whole lot more room (if any), and the extra cabinet/driver size does make a difference when it comes to bass.
The disadvantage is usually expense and transportability (weight/size w/out stands). But hey, I've transported my huge Infinities in the back of my Golf...
Of course, you could always supplement monitors with a subwoofer. I'm not really a fan when it comes to music though.
Definitely look on Audiogon - and Craigslist. For speakers, Craigslist is especially good if you live in or near any big city. You can always drive farther too - I went over 1200 miles for one of my pairs of Infinities...
I'm a big fan of the vintage Infinities, if you couldn't tell... The EMIT tweeters are insanely good, and are on almost all of their speakers. They don't generally get along well with (just) tubes though - all the best (and biggest) ones need gobs of power. One thing some people do is put tubes on top and have a big hulking solid state amp (Krell, McIntosh, etc.) on bottom - I've done similar on a budget with a Carver on top and an Adcom on bottom.
I guess my point is you don't have to limit yourself to just tubes - find a pair of speakers first as they're much harder to find (used, which is the only way to buy IMO) and transport. The used market for amps is a lot easier to navigate.
Edited by BlackbeardBen - 10/2/10 at 9:39pm