McIntosh C50 preamp having a hissy fit! Help!

The setup is: Esoteric K-03x CD player, McIntosh C50 preamp (both plugged into a Furman Elite DMI 15 power conditioner, Synergistic Research power cable for the C50), Pass Labs X350.5 power amp (plugged into wall outlet), KEF Blades.

The problem: the Uni-Q drivers emit a low-level, but angry hiss. Not audible perhaps 18 in. away from speakers. On rare occasion a tiny "tik" could be heard while playing a CD. I traced the source to the C50 preamp -- shut off, no hiss, even though the power amp remained on. I switched out a couple pair balanced cables (had been using Acoustic Zen Wow) -- no difference. However, with unbalanced RCA interconnects the hiss was mitigated by perhaps 80% -- you have to put ears up to the speaker to hear a constant flutter and hiss. But there is also a short "boop" that comes from the right channel upon powering up the amp (after C50 has been powered up) -- also never happened before.

I had a completely silent background with this configuration (incl. balanced Wow interconnects) with ML Summit X speakers.
Any insights on what's causing / how to eliminate the noise? 

Many thanks for your kind and helpful insights.
If it all worked fine then went south, it sounds like an amp problem. I had a tube amp that started to thump on turn on and it needed to be sent in for repair. I would suspect the amp.  
I've gone back to the balanced interconnects -- no initial thump. The hiss stopped when pre-amp powered off, amp powered on, so does not seem to be an amp problem.
OP I'm not trying to be a downer, but I lost a channel on my C2500 it is probably newer than that C50. I think I caused the problem though. I had a cable drop. It took out the poweramp right monoblock. 2 months later the right main 12AX7 (one of two) blew and took the channel with it on my C2500. Something to do with the auto bias.. BUT none the less Macs do mess up every now and then..Even if I did help it along.. :-(

You have a second set of XLRs to try out... I've had them mess up a few times also.. No spiders inside making a new home? :-) Take a peak..