Sub Cross Over Help

Need a little help from you guys running subs. Let me start off with what I have. Power amp, Krel FPB400 CX, KCT pre amp, Bryston CD player, B&W 804s's with a Rel R328 sub. I've been doing a lot of research and reading and learned a great deal from you guys on this site. As of right now I have the sub crossed over at 40hz. The sub is next to the left speaker 18 inches from the corner and 8 inches from the back wall. The speakers are 10 ft apart and a fot from the back wall with my 55 inch plasma TV in the center. My listening distance is 10 ft from the speakers and the room size is 17 x 14. I've used a RTA with a 40hz tone to match the sub with my full range speakers. Everything seems to sound extremely well. The imaging and sound stage is superb. Only thing I've noticed depending on the music I play. I sometimes have to turn down the gain on the sub by 5 clicks, which I'm sure is normal. The gain on the sub sits at 16 clicks from zero. Let me know what you guys think and what I can do to improve my system if any. Please let me know.