the antenna I am using is an indoor amplified antenna, I can't remove the preamp
However, reducing the gain on VHF did the trick, I do receive now exactly the same channels (30 total) that I receive when the built-in tuner of the Vizio works. I have a Winegard antenna in the attic (no pre-amp), which I use to feed the Hauppauge TV tuner card in my main computer (I use it as a second TV but especially as a DVR). With that I receive only a couple more channels, so I am pretty happy with the set-up, considering the difference in height between the indoor amplified antenna on the ground level of house and the one in the attic two stories higher.
I will see if in the next few days I can take snapshots of the setup screens and OSD. The graphic is quite decent, for sure much better than the Vizio's! I really like the "additive scan" function and the real time signal bar that you can call up as part of the OSD, together with the channel and program information at the bottom of the screen.
My only wish right now is that it had a "proper" TV guide. The one that the PHD-208 has is the same as the Vizio's. It shows only the guide for the channel you are watching. You can "flip" through it changing channels with the guide on the OSD, but a "stand alone" guide would have been nicer, like the one I have on the Hauppauge tuner through Windows Media Center.
As for opening it, I will wait till the warranty expires, you never now. I work for a semiconductor company and I am definitely curious too to see what's inside