Try,
1)setting all speakers to small, and running both subs off the LEFT output(LFE off). I would start out by placing both subs near one another...in the best corner of the room.(although experiment with placement as much time allows too).
2)patching the 18" servo to the LFE output, and run the BW off of the sub out. As Shawn suggested.
It might not hurt to see if the local BW dealer would let you audition another BW3000 for a weekend. You could try the 3000s out on the Bass Enhance stuff, and still have the HGS18 for the LFE. Might as well play
TV