The Quads are VERY difficult to mate with subs, because most subs cannot keep up with their purity and speed. I've tried Entecs, but their cutoff frequency was too high and the X-over well to put it mildly ruined the Quads sound. So NEVER, ever use the high pass out to go on to your Quads, rather let them run all the way and find a subwoofer which cuts off sharply at 30 hz, which is the solution, which I found. Was a bitch to place it right, but then the thing worked. Bobby Carvers Signature, you wouldn't believe it, but never, ever over 30 hz. Gradient had a sub made especially for the predecessor of the 988, which was quite nice, with the Quads placed right on top. I also used old Maggie bass panels with the Quads and let them run until about 100hz. Got a beautifull bass and cello sound that way.