If, however, you cannot hear the ground aircraft when you open your squelch, then I would suggest making sure the antenna is as high up a possible, and is free of obstructions toward the direction of the tower.
In the case of my feed, I am about 8-9 nm away from the airport and there is a hill between my house and the airport that prevents line of sight to the field. I have the antenna mounted outside in the highest corner of my house, about 35 feet up, and I am using the very thick coax cable from the antenna to the scanner. My feed can pick up larger aircraft on the ground (UPS cargo B757s and FedEx A340s, and most of the JetBlue A320s - we are a light-to-moderately busy class C airport, served mostly by regional jets) and occasionally, which seems due to weather conditions, I can weakly pick up small GA aircraft. However, it is inconsistant enough that I don't bother to scan the ground frequency.
It looks like this preamp, especially at the frequencies you custom-ordered, would be beneficial to most feeds not located on airport property, as there is always the case of the far-away aircraft.
Which of the custom pre-amps did you order, the narrow tune or the broad tune? (And what is the difference between the two?) Also, after a quick read of the web link you provided, it looks like the preferred location for this pre-amp is actually up at the antenna. How is the pre-amp powered? An internal battery or external A/C power?
When it comes to radio reception, I too am a newbie, so welcome to the club.