No questions on the M70. How 'bout S54?

My philosophy has always been that a speaker that does music well can do HT well, but not vice versa. If the JBL's are great at music reproduction, they will excell in HT duty. Jocko's idea about a DIY center is actually pretty inspired. You can find JBL drivers all over the place and it might be a fun project. Otherwise, check with Harman and see what they suggest. Just make sure you get a spl meter (most use Radio Shack) and test disc to calibrate your rig. All of those speakers are going to have different efficiencies and like someone said, the JBL's are probably going to be waaaaay up there. If you don't calibrate this sucker, the mains will simply overpower everything else.

Good luck and have fun. Building the rig is almost as much fun as enjoying it .