To follow-up I've returned the MANT510, since it can't hold the gain setting. It must be a design flaw or something since others have experienced the same behavior.
I've also tried the MANT310 with not good results. Found it to be sensitive to location and not very stable reception.
Finally, tried MANT410 with very good digital reception, no repositioning, and no problems with the gain falling off. In fact, I believe its performance is better then the MANT510. The MANT410 has separate UHF and VHF gain settings; is available at WalMart; and slightly cheaper then the MANT510.
With the MANT410 I am able to get a digital signal over 20 miles away without repositioning the antenna (although the picture is broken up). With the 510, I'd have to move the antenna near a window and face it in the opposite direction.
I am happy now. Of course, your mile may vary.