[majoconz]

What radio are you connecting to the Spectrum Loop antenna? If other radios work well with other loops, why not try one of those radios with the Spectrum? If you still get the overloading/cross modulation then the loop amplifier is the problem - no amount of filtering or radio gain reduction will get rid of that - but reducing the gain of the loop amplifier will - unless of course there is a fault with the amplifier itself. At a pinch you could try Julesomega's idea of the 100 ohm pot, but put it across the loop with the wiper to the amplifier input.