I just picked up a used e90 with an Akra EVO. I too hear the same RPM dependent, bass, drones. At certain RPMs cruising, it's non existent. But at startup, at low RPMs during initial roll from stop, it sounds like I'm in a bass speaker.
I'm all for great and even loud exhausts, but this one is only loud inside the cabin. Well outside when leaned on... but you get the idea.
I don't have access to a lift, as soon as I do, I'll have to check the mounts. But something seems up.
OP - did you ever sort yours out?