FWIW as it might not be the same animal, I have a five year old W4S ST1000 and it could play Maggie 3.6's loud but with no control of the bass.
It had trouble with Thiel CS3.6's and was a joke with the CS5's.
In my set-up, a Coda CS (300 WPC 8 Ohms, 600 wpc 4 ohms) did much better but got constrained/harsh with the CS5's at higher volumes.
My current amp, a McCormack DNA-500, is effortless with the CS5's. Any change now would be to eliminate my perception that I think I still hear a bit of grain or there is a lack of the sound coming out of a black silence. Based on listening to a X350, Pass might be better but they are hot and huge.
Again, FWIW.
Thanks for listening,
Dsper