Your best solution is going to be to run a line just for OTA.JimTed said:
The built in diplexer in the SWM8 is not "top of the line"
So, if you insist on diplexing, best to use an external diplexer to bring in your OTA, then of course one to break it back out at the receiver.
From there you can start fiddling around with amps/pre-amps.
Also keep in mind that diplexing is not approved or supported by DirecTv with the SWM units (despite it being built in, wouldn't surprise me to see it removed on a later revision), your on your own support wise.