[Kenwood] 940 RECEIVE below 500KHZ

[Joe]

When I tune my Kenwood 940 just below 500KHZ, I hear a click and the receiver seems to become much more sensitive. It also seems as if the AGC turns off below that frequency. This receiver picks up the airport beacons better because of this than any other transceiver I've heard. Can anyone tell me exactly what's happening with the rig at those low frequencies? Is there a pre-amp circuit that kicks in to help receive those  beacons better? I don't think there's anything in the manual about this. 
    Thanks and 73, Joe.