Amp for Revel Gems

I recommend the Boulder 2060 or Soulution 710 - I am sure you would find either to be a significant improvement over the MA6900 :)

Still, it would help to know what you feel is lacking or in what direction you want to head, as well as your budget.

Auxetophone, Thsnk you but Boulder and Soulution aren't the right Solution for me. When I spend 50k on something, it better appreciate, not depreciate.
I do not feel anything lacking. I like the speaker/sub/room combo very much and would like to improve on it, working backwards.
I already have the (very good) C2200 McIntosh pre (Mullards and Telefunkens) sitting unused which more or less tells you the budget level I'm at. 3 to 6 k for an amp I can rationalize. Too much diminishing return TO MY EARS above that price range.
A tubed pre with solid state amp is what I'm repeatedly drawn back to.
I love the tube amps but they're like "divas" and I don't have time for that at this point in my life. As it is, I live in a house full of women.

As a former owner of Ultima Gem, I would recommend two options. The easiest way seems to be following McIntosh way and buy power amp (stereo or mono) for C2200. Or try one of Mark Levinson amps, 436 for ex.
Good luck !

Odyssey Stratos?
http://www.odysseyaudio.com/products-stratos-mono.html

Trying to find a $50K investment that appreciates in value is no easy thing these days, so why not enjoy it on the way down :)

OTOH, I see a pair of Theta Citadels offered right now on Agon for $6895, which I expect would work very well, though they are a little over your budget.

Mark Levinson or Jeff Rowland, and of course higher-level McIntosh solid state amps all seem like good ideas as well. McIntosh and Revel seem to play well together.

Audio Research D400 MKII ??????

I highly recommend the new Mark Levinson 500H series amps. That's what I own powering my Revels.