This is the very important CAST technology update to the original FPB-350M monoblocks. This series was later on superceded by the X-series. The FPB-450Mcx replaced the FPB-350Mc but they are exactly the same amplifier with the same power supply and amplifier modules. The only different is a new driver board that drives the output stage more aggressively, netting 450W instead of 350W. This is a testament of how overbuilt the FPB-350Mc was. The FPB-350Mc is much bigger and better than the FPB-350Mcx, which was essentially the same as the FPB-250Mc that it replaced. Trust us, we know these amps intimately. MSS HiFi has sold more Krell amplifiers than most Krell dealers.