exhaust drone/harmonic resonance

I had a set of Walker Super Turbo Mufflers on my 72 Demon
with an aggressive solid lifter cam. The exhaust diameter was
2.5 in diameter.I had the mufflers mounted at the back of the
car with 90 degree turn downs at the rear axle. I cut the turn
downs at a 45 degree angle so the exhaust would blow out the
rear of the car.I never had any drone, even before i cut the
turn downs. I think i would cut out the x-pipe and see how
sounds.It seems most of the replies concerning a drone have
one thing in common, an x-pipe. It would be worth a try.
The turn downs were directly on the outlet of the mufflers.