I used to tune my resos higher than my batters, so it would make that "tooowmmb" sound. But I had problems doing doubles because the batter head seemed to vibrate in an uneven way. I think it's my choice of pitches, but I wasn't one to really think about it deeply.
Now I tune my toms' batter and reso heads at the same pitch. It sustains better and for some reason my sticks play better. I guess it's the match in frequency... the heads now vibrate more predictably.
Haven't tried the resos to be lower-pitched, although ^^^legnacilyz' link says it's a deeper sound. Although I don't imagine a pitch drop happening... the reso head supposedly sounds out first when you hit a drum (thus a pitch "drop" when you tune the reso higher than the batter). I'd imagine a pitch "rise" instead, which I'm really not comfortable with.
Anyways... it's always fun to experiment... if you have the time (sigh
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