So far I've used an AVA gen 1 FetValve CF, currently using my old AVA Ultra EC Hybrid Pre after rolling in some 6922 tubes in place of the original 6N1Ps. Swapped for 2 reasons: a) wanted to hear the Ultra with the upgrade tubes, b) the Ultra has a tube phono section while the FV CF has the gen 1 Vision phono. Each has it's character, I tend to prefer the tube phono. Mostly I use the DVA 225s in the summer, doesn't seem the most efficient use of energy to run my tube amp while simultaneously running an A/C to cool the house. Another couple of months I'll likely switch back to the tube amp for most of the winter. Both connect to the AVA R2X to convert RCA to Balanced, so can't really address the 2nd part of your question.
In answer to part 3 of your question, I also run a Modwright Modified Pio Universal disc player for CDs which has his tube mod, and have swapped in my old AVA Ultra Hybrid DAC for my AVA DAC5. Again to hear the difference with the 6922 tube upgrade. So, "yes" I have a few tubes before the M225s. I'd think the M225s would pair easily with most any good preamp tube or SS, and give a very satisfying result, fwiw.