Hi.
I've an A4, B8, 3.2 Quattro, on which I recently had the exhaust modified.
Initially I was going to remove the two smaller resonators, mid system. But after further research I decided to leave the resonators on the car, but remove the OE silencers and in their place were fitted a small straight through silencer each side. I too now have a dreadful drone between 1800 and 2700 RPM, in the higher gears.
I've applied some sound insullation to the rear floor, under the carpet, but to little effect.
My next step is to try to attach a 500g mass to each new rear silencer, hoping some inertial damping will erradicate the drone/resonance.
If I get any significant results, I'll try to post back.
Sorry I'm not much more help, perhaps my experience will be food for thought, for others who have not yet gone this route, but are considering it?