I too had a 1997 vintage FPB 300 and thought of upgrading it to the 400CX. As you know the 300 can be upgraded to be the sonic equivalent of the new 400CX but the chassis would still be the original unless you paid for a complete chassis too. In making my decision I looked at the economics and felt that upgrading was not a good move financially. The current value of an FPB 300 is $3500 and the current value of the FPB 400CX is $5500, as per Audiogon ads. The cost of the upgrade is $2K and doesnt include the new chassis nor does it include the round trip freight back to Krell. I felt that the money spent on the upgrade would most likely not increase the overall resale value of the 300 by nearly as much as it cost me.
If you do decide to follow the upgrade path heres a brief description of what I was told happens.
There are two upgrade's available for the FPB 300. The CAST upgrade and the X upgrade. The "catch" here is that you cannot get the X upgrade unless your amplifier is up to "c" or CAST version level. Meaning that certain elements of the X upgrade cannot be installed unless your amplifiers internal circuitry is at "c" or CAST level.
The CAST upgrade. This upgrade replaces the driver board (the 2nd largest PC board on each amp channel) with a superior sounding version of this board that also allows Krell to access the amplifiers full CAST (Current Audio Signal Transmission) capability. Krell also installs CAST input connectors on the rear panel. Even if you are not operating in CAST there are still tremendous sonic benefits from this upgrade even when operating in balanced (XLR).
The X upgrade. This upgrade increases power output an additional 100 watts per channel in addition to increasing bandwidth (from 20Hz to 100KHz up to 20Hz to 400Hkz). The added power and increased bandwidth combine to provide you with even more of why you bought your Krell amp in the first place. When all is said and done your original version FPB amplifier will now be (at least sonically speaking) identical to a brand new "x" version FPB stereo amplifier.
Even though the 300CX only has a 2KVa transformer versus the 4KVa transformer of the 400CX (and original FPB300) I found the new 300CX to have far greater extension and control than my original 300. It truly sounded better in all regards, better bass extension, more accurate and detailed sound staging, better midrange and treble presentation as well as more upper end detail activity, so I replaced my original 300 with a 300CX. One thing that will benefit your Krell amp is to run it into a dedicated 20amp main. They love the current.
An expert I'm not but I hope you find some useful info here.