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.
