I too think it must be the amp but this test will tell for sure.Hopefully Kosst will take my response to this in the constructive spirit in which it is offered: If the problem goes away when the amp is changed, while it would substantially increase the likelihood that the Pass amp is the culprit, it would not by any means be a conclusive indication. It could simply be that the other amp is less sensitive to the effects of a speaker cable having very high capacitance.
Also, can you answer a couple of questions I had raised earlier:
1)What is the full model name of the Ortofon Black cartridge?
2)Do the OCOS cables have a "red" termination network, a "black" termination network, or neither.
Good luck. Regards,
-- Al