Don't forget the Mk1 has differential ouputs: it sounds as if you haven't fitted the short earthing leads between the head unit and the amp that were supplied with the player. You need to earth these near the amp, so you should fit them at the amp end of your cabling, rather than near the empeg. Some people have also reported an improvement in noise isolation when the sled itself is earthed.
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One of the few remaining Mk1 owners... #00015