I resolved it by not using the diplexors. I put the antenna on top of my tv and got reasonable signal strength, much to the disgust of my wife. Later I ended up running 2 more RG6 lines to my media center to support my 921, so I put the antenna in the attic and got even better signal.
Now I'm adding some more receivers, one of which will be a 510, and I'm trying to decide exactly how I want to set things up. I'd like to feed more than one tv with the output, but maybe with a switch box so the tv's can watch either what the 510 is showing, or what their own receiver is showing. It's just an idea I'm kicking around. I plan to replace my lnb's and switch with dishpro/dpp44 so I may bring the 921 output back up to my closet distribution center too (that was an option I added when they built my house) and add it into the mix. I'd hate to record something and then not be able to watch it on the tv I want to see it on. I also want to do this without running any more cable, which is where the diplexor idea comes in.
At any rate, the sony diplexor says "DSS" on it, and everything I read about it says it's for Direct TV installs.