There may well be tweaking to do on the DSP parameters; it will "dim" out the audio if it thinks the signal strength is getting too low for good audio quality. Possibly, this is set to be a little too sensitive.

Check that 12v is going up the antenna control lead if you can (the tuner module has a current limiting cut-out on the wire) - it will supply 150mA so this shouldn't be an issue for an antenna amp.

Hugo