The first obvious thing to do is remove the distribution amp and place an amplifier at the antenna mast. That should improve your SNR by 1-2 dB.
One VHF low antenna that I know of is the Antennacraft Y526. Or you could go with a 2-13 antenna like the CS600 and limit your ANT751 to UHF signals.
http://www.antennacraft.net/Antennas/AntennasVHF.html
Not sure how you would keep the ANT751 for VHF high and UHF. Devices that I know of split VHF from UHF.
I'd use a RCA preamp
http://www.amazon.com/RCA-TVPRAMP1R-...dp/B003P92D9Y# and connect the VHF antenna to the VHF port and the ANT751 to the UHF port. Replace the CM distribution amp with a passive splitter.