There was a problem with the power supplies on some preamps and soured components. They used 2 6h30s in the power supply which not always aged at the same rate. This could affect the sound. So ARC offered an upgrade to replaced the 2 6h30 tubes with a single 5771. I know my CD7 was upgraded this way. I don't believe the Ref 5 had this problem Though.
I still think it is something simple. I would try moving the preamp to another room and use a different power cord. Just to see what happens. If that fails I would call Kal at ARC. If you bought it from a dealer you could have him take a look at it.

