Yo, Sfstereo, the answer is yes, yes, and no...I don't know. Yes I have heard them. The difference in sound is not subtle, it is the biggest improvement I have heard from Krell in the whole time I've known about Krell. The new FPB-xxx-Csx series is smoother, more laid back, and ballsy-er then the former cast series. They are much easier to listen to.
I passed on the chance to own the former FPB-300C (at a great price) and am now planning to buy the 350Csx (stereo, not monoblocks) because it is way better.
No, I don't know the difference in price on the ones you listed, but I don't think that the prices changed much.
If you are interested, I know where I can get a new demo unit (FPB-350Csx) at a good price.