My system is a twin 2.25 inch with 1 short muffler on each pipe at the rear of the system. I get no drone what's so ever and the note is absolutely f/n awesome
. My car is also a BA XR 8 but is a Ute.
You might want to go a smaller pipes or relocate the mufflers. The second of the 2 would be the cheaper and possibility better for performance.
other then that there's not a real lot you can do. I would also get another exhaust fitter to rectify the problem as the one you went to should know that what he was doing would cause droning and advise you of it and what to do about it.