I originally had a diplexor on both of my lines going to my 921, one for the OTA Antenna feed along with the satellite feed, and the second being a backfeed to other rooms in the house (of course along with the satellite feed). I have since gotten a DPP Twin and have one line just dedicated to my OTA Antenna and the second line with a diplexor on it for backfeeding to other rooms in the house.
Saw maybe a 5 point increase in my OTA-HD channel strength on my 921. Was hoping for more, but when I had a few channels hovering around 75-80, getting them above 80 helped to keep the tuned in much better (as well as occasionally being able to pickup a couple of my weaker channels that used to be in the 65-70 range).
But, like Simon says, can't help ya much unless you give more details on your setup. LNB type, receivers, switches, etc.