Audio Research CD6 vs CD9 original and SE versions

I recently had an ARC CD6 in for a demo.  The system is:

Intel NUC --> MSB Analog (USB) --> Krell FBI --> Thiel CS2.7

With the ARC installed it went straight to the Krell.

What I heard was a fabulous top to midbass.  Where it did not sound right was in the bass.  For lack of a better term the bass sound was "rounded".  I tried a few different cables with varying degrees of success however I could not live with the bass.

My question is does anyone have experience with side-by-side listening of the CD6 vs CD9 and or the SE versions?  If so what differences did you hear?  I listen to mostly classical on this system and having poor bass drum, tympani, and double bass performance won't cut it.