Personally, I would view the "not all watts are equal" mantra with caution.
I'd bet against a 20wpc tube integrated working as well as 100 watts solid state (or tube, for that matter) for your 91db speakers, all else equal. In fact, I wouldn't go near 20 watts for your speakers, no matter how "unequal" they were claimed to be (complications noted by ghdprentice noted).
That said, there are tube amps with more juice in your budget. Rogue has already been mentioned, and gets great press. I just bought a 60 wpc Jolida/Black Ice used here that sounds lovely, and something like that could be had for way under your budget (see Walter at Underwood for these).
But my sense is many of the tube amps will not have all the functionality you want. If you are willing to consider a "just an amp" (an approach I'm attracted to), I'd throw Pass SS stuff in the mix with the toobers.