To narf and wfi,
Yes, please do try the Terk HDTVi, and make sure that there are no amplifiers anywhere in the setup.
Also, if you do have a powered antenna or anything else with a built-in amp, simply unplugging it will NOT work. In most cases, when you take away the power from an amp, the amp becomes a dead circuit that doesn't allow anything through. You cannot assume that an amp will become a "pass through" device when the power is removed. The best thing to do is to make sure all amps are completely removed from the setup.
The signal analysis for narf shows lots of channels with lots of power. Even with an indoor antenna and no amplification, it might still be possible that there is too much signal power for your receiver to handle. If you are still missing most channels with the HDTVi, then you might want to try adding an in-line attenuator (like
this). This will lower the power across all channels and hopefully help your tuner cope with all the strong signals.