I've fitted a Grace 707/Sumiko Oyster to my 1974 AR XB with rather good results, but there is now more hum than when I first started using it. Would an auxiliary ground line from motor/chassis mount to amp help? I'm using vintage Dynaco SCA-80Q. This is set up as LP to computer to CD, although it rivals my Linn system for listenability. Any thoughts?
Sounds like it could very likely be some kind of ground issue. I'd check all grounds and connections, and it wouldn't hurt to ground it like you mentioned. Also, keep all signal lines well away from any power cables or power supply transformers.