I use the OTA input on my SWM8 and diplex out at the STB/TV and have no problems. I have seen some reports that if you have stations in the VHF-Lo band there might be problems, maybe that's why I don't have any issues since my lowest OTA station is 7 which is in VHF-Hi.
BTW, the diplexer is a Channel Master 4001FD Diplexer.