Why don't you set the rears at 80hz and see if it helps any. If not, then you know 150 is fine. I don't think changing to a direct drive will change the issue you are raising, but that is over my pay grade.

Panny 3000 PJ, 118" Carada, Denon 3300, PS3, Axiom QS8, PSB 5T, B&W sub, levitating speaker wire