Krell continually improved the FPB series over time and the part number evolved along with the improvements to the basic design. There are basically 3 stereo amps and 3 Mono's as follows:
Stereo - FPB200,FPB200C,FPB300CX (Transformer 1700VA)
Stereo - FPB300, FPB300C, FPB400CX (Trans 3000VA)
Stereo - FPB600, FPB600C, FPB700CX (Trans 2X3000VA)
Mono - FPB250M,FPB250MC,FPB350MCX (Transformer 1700VA each)
Mono - FPB350M, FPB350MC, FPB450MCX(Trans 3000VA Each)
Mono - FPB650M, FPB650MC, FPB750MCX (Trans 2X3000VA Each)
All the FPB amps were known to put out far more than their given rating, for Example the FPB350M would easily put out over 470 watts stock into 8 ohms. The "C" amps added a few driver improvements and the CAST connection (Input Board) in place of the single ended connection. The C amps had a noticeably improved mid-range presentation. The "X" amps were a complete redesign of the driver board with added capacitors and further improved drivers for a sizable increase in bandwidth, Sustained Bias improvements, Higher "rated" output closer to it's actual output. The X amps had tighter and faster bass, smoother more liquid mid-range and an obvious extension in the treble with increased clarity and imaging, the treble was more extended but more natural and effortless. The "X" series was also more dynamic. I currently have 1 FPB200C, 1 FPB400CX and 2 FPB750MCX amps. I modify these in several ways, the design is terrific but now days there are some higher quality better sounding parts available. I can make these amps sound better than the originals not to say anything negative about the original sound but mine are much smoother, more dynamic and far more detailed, the most amazing part is how neutral and natural they have become, they are so balanced sounding now. The treble is even more extended but sweeter and the mid-range is very natural sounding, the bass is stronger and more extended too, but more effortless. The bass initial attack is a little stronger but you notice more how it extends naturally past that initial attack. My FPB200C sounds much more effortless, detailed, it just does not do anything to call attention to itself, it's smooth and graceful no matter what is going on where the original could get flustered at times and lost focus. My FPB200C sounds more like a FPB450MCX but more articulate and light on it's feet, it has a top to bottom competence that is just amazing.
I use the Apogee Diva Ultimate as my reference speaker from Rich Murray. I think Rich's speakers are the best on the planet or the best I have ever heard if you have the room for them.
With the larger amps you are getting what you pay for, more of everything, drivers, caps, transistors and so on. The circuit boards are pretty much the same mono or stereo but the Transformer size is DOUBLED for the mono set-up, plus the increased separation from completely separate supplies and you do hear this easily.
Keep in mind people on here compare amps that are different ages with different states of health. For FPB amps the wall power is critical, 220V is ideal with a 20-30 amp circuit. I run a Torus 100 transformer with individual dedicated 20amp circuits for each amp on 120V(Each circuit can handle 40 amps Short term). The detachable power cords are a common weakness of the earlier amps, the cable manufacturers are the ones that want detachable cords, the simple direct (attached) stock cord is the best I have heard. I hope some of this info is helpful to you, good luck.