KRELL FPB 650M MONO BLOCKS ..?

Banmac, try this first for no money outlay. This is the way I would use it, and it will be very different.

Leave your preamp out altogether, and go direct, as it looks as though your Esoteric sac x-01 has a volume control built into it looking at a picture of the remote (read your manual).
Try using it (the X-01) direct into the FPB-350 and using the volume control it has, it will still be a perfect match.

As for your 350m amps you have, they would be walking in the park driving those speakers, and quite capable to over drive those speakers you have.
I can only think of one speaker where the 650m would be better and that's with Wilson Alexia's as they are a pig of a load down to .8ohm epdr in the bass, and even then it would maybe just be better.

Cheers George

As tested by Stereophile your 350M's can give.

"maximum power delivery of a monstrous 475W into 8 ohms (26.8dBW), way above the specified power. 850W was available into 4 ohms (26.3dBW) and while I measured 1060W into 2 ohms (24.2 dBW)"

I cant see any improvement happening by going to the same topology even more powerfull 650M's driving your 90db speakers, as both amps would drive the 801n speakers on their ear.

This is why I ask what are you trying to get that needs to be different, and what is your source and preamp?

Cheers George

"I would like more power"

I don't think you would gain anything at all, can't believe you don't have enough power already.

What are you trying to get that you can't get with what you've got already, better bass, sweeter mids and highs, change in tonal balance?? list your source and preamp as well.

Cheers George