The use of an outside, mast mounted pre-amplifier will only help on three possible cases:

1 - The co-axial cable run is long ( even 50 feet at UHF frequencies is long) and attenuates the rf signals before it reaches the TV tuner

2 - The TV tuner requires a larger signal due to its possible sensistivity levels (some set or digit boxes require more signal than others)

3 - A splitter was inserted along the co-axail cable to feed more than one TV

However, if the HD antenna is blocked, no signal or multipath, a pre-amplifier may not be much help.

Also, keep in mind that with an amplifier deep rf fades will result in more possible outages.

My suggestions, with the low cost of these amplifiers, a trial and error method is worth it.