If it's that irritating I'm sure you're not the first to complain. Have you spoken with Flux to see if they can fit a mid range resonator to counteract the drone?
You can see the resonator in this
IPD exhaust (centre section) and there's also an old thread on the same subject over on
Volvospeed.
I'm almost certain the original Volvo exhaust on my old T4 had a great lump of metal attached to it post cat...like a dead weight...to counteract vibration from the pipe I always thought. Clan will be along shortly to correct me one this