Practice mufflers

I'm running a pair of Borla Pro XS on my 318 Dart, along with fulllength Dougs Headers, a 2.5" mandrell X-pipe and tailpipes to the end.
They rumble nice and quiet at idle, but get just a bit too loud nearing 2000rpm with some throttle.
Once off the throttle they're fine again.
I can also think maybe the X-pipe made the mufflers sound more louder and monotone now, instead of a more alternate, stereo sound like with seperate exhaust pipes.

Then again, my ('73) Dart is fairly loud inside and I may need to do some more sounddeadening in the drivers compartment.
I chose the Borlas because they were offered under halfprice to me and they flow as one of the best out there I recall.