I just looked at the manual for the Stacker, and it looks like he is using the stub method instead of a UVSJ. It is a popular method that is used in many combo antennas, but Winegard uses a combiner PC board that is similar to a UVSJ.
The stub method is simple and inexpensive. It shorts out any UHF signals coming in on the VHF antenna. The downside is that you need several sets of stubs of different lengths to cover the UHF band.
I suggest you try that modification to see if it improves UHF reception of your marginal channel, but keep in mind that OTA signals are constantly changing in strength.