It doesn't power the HE-560 fully? That's weird, it had enough power for my T90 and K7XX. Interesting...

The HE560's relatively low sensitivity still makes it a bit difficult to drive, although not nearly as bad as some other planars out there.

I'm a little thrown off by the power thing and the bass thing too.  Did you have to crank the volume knob of the X1S to 3:00 or more?  Also, take a look at the measured freq reponse, it is ruler flat to below 20Hz.