If you want something better than 300cx go with a bigger Krell, either 700cx or older 600C or 350Mc monoblocks. Anything else will be different at best.
I would suggest staying with 300cx and trying a different preamp. KAV is ok but clearly a budget product. Simaudio P-5, Bryston BP26 or Pass X1 are much better and can be found for around 2-2.5k.
I have owned three FPBs and they really show shortcomings of associated gear. Once the rest is up to snuff your Krell will surprise you.
I would suggest staying with 300cx and trying a different preamp. KAV is ok but clearly a budget product. Simaudio P-5, Bryston BP26 or Pass X1 are much better and can be found for around 2-2.5k.
I have owned three FPBs and they really show shortcomings of associated gear. Once the rest is up to snuff your Krell will surprise you.