Not yet, but I am planning to. I own the FPB-300.
The FPB300 sounds much better than the FPB200 (and the previous KSA250 sounded much better than the KSA150). I do not know if the upgrade of raising the power of the FPB series to the new "x" series is an improvement since the power supply remains the same. It may just mean more power with no change in the quality. I do not know.
I believe the changes to the FPB amps do not actually involve more ouput power. The amps are so conservatively rated that Krell can easily label the new models with a higher number. The improvements to the amps are in some new CAST circuitry and a new Sustained plateau Biasing scheme. This is the info i have come across, mebbe someone from Krell would like to chime in and clarify it further for us.