I suspect it is an issue with the antenna match to the receiver. One might provide better performance on any given antenna than the other. Which one does better depends on the antenna. And nothing says the better one cannot be improved upon with slight changes. This is down at a frequency range where antennas should not make any difference as far as QRM from outside the station's perimeter is concerned. But things such as wind can setup charges in wires it blows past. Front end impedance at DC can have significant effects on this. It might be entertaining, if the antenna is large (say a full size 80 meter dipole) to place an easily obtainable T connection and 50 ohm dummy load at the input of the receivers. If connecting the antenna raises the noise in both receivers the performance comparison should still face the same externally generated noise issue perhaps with less atmospheric effects differences on the antennas. The AirSpy's noise figure is do low that at any frequency under 10 MHz a 20 dB pad on a dipole antenna's feed line should not materially affect received SNR. So something else has to be creeping into the setup. Rain static? Wind effects? Really close by really high power AM radio station? That latter could exceed the AirSpy's dynamic range before it would affect something with an extra bit or two of A/D resolution. An AM band reject filter would mitigate this problem nicely. And it might improve both HL2 and AirSpy.

{^_^}