Note that I have an amplifier on my antenna pole - who knows, maybe that is having a negative effect on the PHD tuner.
Not knowing your situation, you could be getting overload from that amp from something other than TV stations (2-way, pagers, or a host of other RF that the VRX might not like. Pre-amps overload very easily since they pickup most everything other than just TV signals.
I see a difference between tuners from 2007 and before and tuners manufactured in the past two years. other than that, on this and a three year old TV, I really don't see any difference worth talking about. AFAIC, and already stated, it isn't sensitivity, it's error correction that seems to play the biggest role.
What's the point of buying the equipment and setting everything up, if the external tuner you buy is no better than these settop boxes.
Between the excessive "RF" inside a PC, the fact PC's are a PITA when it comes to video hardware and the problems with driver & software conflicts, OTA HTPC's aren't worth it AFAIC.