fuller sound at lower volumes

I am looking for a fuller, richer, and deeper sound at lower volumes. My current system is a Musical Fidelity a3.2 integrated, a3.2cd player, and Paradigm Studio 40 v.3 on stands. My room is about 18x12x8. I am thinking about a McIntosh MA6500 integrated. Or would adding a good subwoofer be of benefit?
A good equalizer or an amplifier with loudness control.

look into the Fletcher munson curve
A sub would be far more influential in getting a "fuller, richer, deeper sound" at lower volume than getting a new amp.
The subwoofer will do the trick!
I think you should look for speakers that fit your need.
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.
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.
I'm a little confused here. Using me and my situation as an apt. dweller; the bass at low volume is the one thing I don't want. At the right time of day,that is different. You might try listening near-field.(If you don't have 600lb. floorstanders.)--Mark your spots normal and nearfield.
In view of all of the recommendations for new electronic's, let me elaborate on my initial recommendation.

First, if you had full range speakers I would be the first to agree with getting an equalizer or an integrated such as the Mac with both tone and loudness controls.

However, your post said full, richer, and "deeper" sounds. You can't get blood out of a stone. No equalizer or tone control is going to boost a frequency that your speakers don't reproduce. So if you can only do one of the options I would think sub.

Adding a good sub with variable cross over points and volume controls would allow you to get fuller sound at low levels by setting the cross over at its highest point and increasing the volume to suit. For listening at normal levels you would want to back off on the volume and lower the cross over point to eliminate bass boom and deterioration of the stereo image.

Best of all worlds (without recommending equipment outside of your post) would be to get a sub and the Mac. Then you would have a full range system with tone controls galore and an amp which will add to the "richer" criteria you set out.
Yes, the work of Fletcher and Munson, and more recently, Robinson and Dadson, tells the story quite well. The ear is less linear at low listening levels, perceived bass amplitude being diminished. The EQ or loudness contol is one answer. If that is anathema to you, you may just have to just move your speakers closer to the rear wall when you listen quietly.