How are these antennas hooked up?
Are you using the 91XG for UHF and the HD7000R as a VHF antenna? If so, they need to be combined through a diplexor like the Pico Macom UVSJ. Alternatively, you can get a pre-amp with dual inputs (UHF and VHF) like the Channel Master 7777 that takes care of combining signals and boosting them at the same time.
How long is the cable from the antenna to the TV? If the cable is more than 30 feet, you ought to use a pre-amp to overcome the cable losses if you don't already have one.
Also, do you split the signal to multiple TVs? A splitter will reduce the amount of signal that reaches each end-point. A pre-amp can overcome this problem too. If you do have a splitter, but some of the branches are unused, you ought to "cap off" the unused branches with a terminator (like
this) so that the open ends don't "reflect" signal back into the cable, potentially causing a kind of self-interference.