teleview is correct, the HD4400 can be removed from the system, replaced by the higher gain UHF antenna. I had Baltimore and DC directions wrong in my head.
Antennas Direct is now indicating the DB8E is in stock:
http://www.antennasdirect.com/store/...V-Antenna.html
The 91XG is long so at 29' AGL, it will be a bit more challenging to maneuver into place, then it will be affected by wind and ice more than a panel style antenna. That said, it's well built, and would serve well, I own one and don't regret it.
The DB8E is based on a newer design that focused the gain into a narrower range of frequencies (real CH-14 through CH 51). Older UHF antennas are usually designed to include real channels 14 through 69, so they sacrifice a bit of gain compared to a design optimized for a narrow range of frequencies. The net result is, the DB8E will give you about 1 dB more gain than the 91XG... not insignificant, but not a big difference either. The DB8E can be configured as a conventional high gain unidirectional panel (both sub-panels facing the same direction) or as a variable medium gain, mutlidirectional antenna. This feature is not relevant to your application, you want to set up as a high gain, unidirectional... left and right sub panels facing the same direction.
Placing a preamp on each antenna, ahead of the UVSJ eliminates the few tenths of a DB loss in system noise margin caused by the UVSJ (which should have no more than 1 dB insertion loss). If you are at the edge of reliable reception, this might provide noticeable improvement.
You're in a spot that has some issues with co-channel and adjacent channel interference. Your first and best tool to overcome this is a highly directional antenna. The only way to know what will be received reliably is to hang some metal in the air.