fuller sound at lower volumes

Tube preamp and more efficient speakers will breather life into the system at low volumes...of course, then you might want a tube amp better matched to those more efficient speaks...a slippery slope, but a fun one!

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.