Hiss from speakers that increases as loudness control is decreased

I know there are a ton of articles on the web and our forum on hiss and hum but I didn't find anything that could be of help. My gear comprises of a Audiolab cd transport, Mccormack DAC1, Mcintosh C27 and a Mccormack DNA .5 and Vandy 2CE Signatures. They are all connected through Kimber PBJs. There's a noticeable hiss and a faint hum that becomes apparent when the C27 loudness control is at 12'o clock. As the loudness is turned down (towards flat) the hiss gets significantly louder and the hum also becomes extremely apparent. When I disconnect the preamp, the speakers are quiet as a tomb. I have swapped the cd transport with a streamer and it hasn't made any difference. 

I don't know if this is a ground loop issue or an issue with the C27. The last time it was serviced was about 8 years ago.

Any pointers/tricks are appreciated before I haul the preamp over to the technician for diagnosis..

Thanks