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.