mikroski, I tried my Theos with a Consonance M100+ tube integrated amplifier and I was surprised that this combination worked quite well together. Absolute spl's were lacking and there was a slight loss in sparkle, but overall in smaller rooms it worked. Care must be taken though, some lower cost tube amps won't work very well
I'm negotiating the purchase of a pair of mono tube power amplifiers driven by a custom passive preamp to drive my Theos. It's a triode design with 4 x 6550 tubes per channel, with massive output transformers worth $1000 each! - all for a head-banging 60 watts into 8 ohms and 90 watts into 4 ohms! The main consideration is with these power amps is that they will drive a short circuit. So low impedances won't worry them. You get what you pay for with tubes. Don't be afraid to experiment, but it's a good idea to try the amplifiers first before you buy.
Edwin.