Take your toms, one by one, and do this:
Install batter and reso heads as evenly as possible and bring to finger-tight.
Add tension very graduallly, no more than a 1/4 turn at a time, using two keys on opposite sides of the head simultaneously. Tune both heads the same amount.
Strike the batter head in the middle after each 1/4 turn rotation. At first you'll get a flappy, papery sound. Add another round of 1/4 turns to both heads. Eventually you'll reach a point where you get a real tone. Make both heads the same pitch and make the lug-to-lug tuning decent.
That is the lowest note that drum will play.
Taking off the reso may let you tune a bit lower, but not by much, and the tone is terrible. Different heads may allow you to tune a bit lower, but not by much, either. Thicker heads will let you go somewhat lower because they will give a lower tone at a given tension.
If that's not low enough for you, you need bigger diameter drums.
Some drummers keep their toms at this lowest-point tuning, but many prefer to put the drums where they sing the best. That's a little higher than the lowest tone. Keep adding tension gradually to both heads and you will hear the drum bloom, get louder and fatter. That's where the drum sounds its best and where it sounds loudest and punchiest as well.
