fuller sound at lower volumes

Only one of the above (cinematic): an equaliser or loudness control on amp. At low volumes the ear is less sensitive to low frequencies and the upper two octaves.

Alternatively, look into spkr placement and try to couple the midbass response of the two spkrs (if you haven't already). Finally if you want to invest in more low frequency energy you may follow Newbee's advice for a sub. Subs have controls on them. Cheers

The McIntosh 6500 would be an excellent solution in that it has a loudness compensation circuit as well as a 5 band tone control. It's an expensive solution, but it will work.

I just purchased a C-J tube rig for my office system, and
got what your seeking.

That is what the "loudness" button on many receivers is designed to do. It adds equalization at the upper and lower frequencies, to compensate for our loss of hearing sensitivity in the frequency extremes at lower volume levels.

Ditto Onhwy61. The MA6500 would be perfect for you since it has a loudness compensation circuit. My C712 had one and I used the heck out of it. I miss it a lot now. Also, the McIntosh will be much richer sounding than the MF gear in any case.