Hey John,
Having used my ZR2 for a number of years, with three different amps and two sets of speakers, my initial thought to your first question is no, a ZR2 will not limit headroom.  Can you be a bit more specific on what you mean by limiting headroom?  What issue(s) are you wondering about that you think the ZR2 might exacerbate?

Doug, Lon, thank you for getting back to me.
Doug, my concern is that by resolving the bass issue I might push the amp to its already limited limit. Lon confirmed my suspicion of increased compression (in dynamics and volume) but will the sound stage depth and width suffer as well? Will the magic be lost in place of a bit more low end?

Also, the volume dials typically live around 8 o’clock sometimes 9 o’clock 10 is the rare upper limit, and that is where the amp no longer gets any louder. It just begins to compress.

Thank you