This is good to know. I have two RCA751R's to put on outside poll. I am experimenting in attic now. The issue is I have a channel 5 (NBC) west at 9 miles (72-82MHz). The rest of the channels are North (mostly UHF and one Hi-VHF on Ch13), and one (PBS) on UHF East-SouthEast. I am getting it, but my Channel Master recorded programs sometimes have interruptions (in good weather). My plan is to have one pointed 751 WEST and one pointed North East, and try and get all the other channels. The 751 may not be tuned for Low VHF but from my calculations it is almost 1/4 wave. So it does work. However I had the idea of adding length to the elements in the VHF section. Since it is an odd ball Yagi and Log Periodical Dipole hybrid I am a not sure. So I will do a test set up and use my older Sony TV or my hauppauge tv tuner in my old PC (I made into a DVR) and measure the signal and errors and experiment with element lengths. If I want half wave it will be roughly double for channel 5. So now a stock 751 I recall has 39" rear element dipole, it will need to be 69". Since I will have a good signal I am sure outside with a mast preamp (RCA with separate VHF and UHF inputs) I will be fine. I may however can the second 751 and look for a dedicated low VHF. They had several great models in the past, all discontinued.
You might consider the Winegard HD7000R it's similar to the 751 but has the longer elements for low VHF. Winegard brought it back this year but it looks like they might now be in short supply. http://www.solidsignal.com/pview.asp?p=HD7000R
Glad to see you are experimenting and please share your progress with us. You may not need the RCA pre-amplifier and depending on your situation, it may do more harm than good. You can combine your antennas (without amplification) for UHF and high-band VHF using a UVSJ (UHF-VHF-Signal-Joiner) but I'm not certain it would pass Channel-5 / low-band VHF.
You cannot optimize a Yagi-style antenna for one channel by altering the length of only one element, including the reflector. As with most TV antennas, the ANT-751 is a compromised design intended to receive many channels including those on high-band VHF.