Let me first say that I have had 9 exhaust combinations on my car in 18 months.
It's not the x-pipe that is the problem, it's the axleback. The Akra system is quiet but you are right, the bassy note down low can be disturbing. Not to mention, a short resonator isn't going to cancel out those long wavelengths. AND, you won't find titanium resonators or someone to weld them.
I had this problem with the two Gintani axlebacks (sport and hybrid) I had. Volume-wise, they were very similar to my Dinan through the rev range, very reasonable. Except down low. The Gintani exhausts both had a very bassy sound down low. They were disturbingly loud cruising through parking lots but very mellow at high revs (4K+). My Dinan gets louder as the revs increase, as it should, with no bassy note down low.
My suggestion to you is to (at least) get a different axleback. That's a pain since you need adapters to run the Akra x-pipe with a different axleback.
2008 IB E92 M3| BBS | KW | Arkym | Platte Forme A.G. | Active Autowerke | K&N | Fabspeed | Dinan | Evolve-R