Just through head selection. Go with double ply heads such as Emperors, Pinstripes, Evans G2's, EC2's etc. Also, you may want to think about a "controlled" resonant head, such as a Powerstroke 3 (single ply, but built in muffling).

That will certainly help deepen the tone.

Thanks for the responses.

Old Slingerlands should naturally have a pretty deep, resonant tone, in part because of the rounded 30° edges and three-ply shells. If you've got a monster 20" tom tuned low, you should FEEL that thing almost as much as you hear it.

If your drums pre-date modern sizing (roughly 1957), your head selection may be limited to Aquarian's American Vintage heads. Shells built before that time were sized literally -- the diameter of a 14" drum was exactly 14 inches. Modern shells are built about 1/8" under their rated size. Heads constructed for contemporary-sized shells may be a tight fit on old drums, rubbing and possibly binding on the shell.