fuller sound at lower volumes

We've had three versions of Paradigm Studio 100's and it wasn't until good fortune smiled and we acquired a pair of used Avalon Eidolons that we realized just how good our system could sound at low volumes. I think your electronics are fine. I would take the suggestions here and at least demo a powered sub.

BTW we liked the version 2's hands down the best in our room.

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.