Sirquack. You sound like you are almost there on a second sub which I would heartily recommend based on that large room of yours. I don't think you can ever have enough sub headroom. I have an EP600 which I am very pleased with on one centre sidewall and an Ultra on the other side of a 3500cf room. Going from one sub to two subs has made an enormous difference in sound quality in the room and really smoothed nulls and peaks throughout so bass is uniform in any seat. My decision now is whether to sell the Ultra and replace it with another EP600 or buy 4 500s collocating two one each sidewall. Because aesthetics are also a concern I just might go for a second 600.

My recommendation is sell the 350 and buy either 2 500s, 2 600s or better still 4 500s strategically spaced in that large room.

Also I wouldn't place the centre speaker on the sub because passive vibration of the centre's drivers may become audible. I think this problem is worse if the centre has a port. I tried that setup for a while with the centre on the 600 and have sinced moved the sub from below the screen.

John