They sound a bit thin with anything, it is their nature. They sound as good with the CV2+ as any solid state amp that I have used them with. I do like them quite well when I was using them with the Pulse Infinity (often called warm even with the ESS dacs) and my Garage 1217 Ember with a pair of Visseaux 6J5g tubes in it.
Maybe the best I heard them was at the Cavalli booth at RMAF using some $8K DAC and his Liquid Gold amp. Still the basic character was there, but still very, very nice! They do scale well.
Over time I have done lots of playing with the 560. Pad rolling (I like the original Jerg pads the best, followed by Focus A) cables, grill mods and most of the mods that Jerg described in his mod thread. I would guess that most wouldn't want to spend the time and energy to do these things. I have two sets, one stock and the other modded. The modded ones are better to my ears, but really there is not the kind of differences that would make most say that they are entirely different in the FR category.
To my frame of reference the cables made the least difference. Mostly almost zero. Many have said the Draug cables are good with the 560, but I flat refuse to spend that kind of money for wires. I guess that I will never know for sure.
Bottom line is if somebody likes the FR, then they do enough things really well that it may be worth buying them to supplement or replace the 400i, Just beware that the signature is pretty far from musical, fun, or forgiving. It moves much closer to analytical and cold/thin. IMHO, of course.