The frequency band used for AM is not that high (550Khz to 1720Khz). The Spectral preamps of that age were designed with ultra-wide bandwidth, but I assume had some form of RFI filter on them to prevent this kind of interference. It's possible that a capacitor in this circuitry has gone bad which is now allowing these signals to be picked up and modulated the output signal.
Spectral preamp picking up a radio station
I have a friend with an all Spectral pre/amp system (25 years old). When I was listening to his system, I can hear quite distinctly some radio station playing from the left speaker even though there is no radio on in the house. We were playing an album, so it can't be the DAC. This started happening within the past year or two. Could a malfunction in the preamp cause this?