HK Citation 7.1 & 5.1 "pop" when turning on or off

I would start by cleaning all your RCA plug connections.
Does the pop happen on all speakers? If so, start at the receiver or preamp. You might also try swapping IC's out too, just remember to power down first.

I have a Citation 7.1 and there are relays to control power up/down, so very unlikely it's the problem.

It does happen on all 8 speakers (not the sub).

I tried using a few different ground loop hum eliminators from the preamp/receiver to each amp, but that added more constant noise in the background.

I'll try some deoxit. I'm using bluejeans cable interconnects, but will try another brand and/or roll my own.

I do have all three amps rackmounted, at the base of my rack, with a couple inches on the top/bottom. Any chance they are interfering with each other?

I also have tried hooking them up direct to a dedicated outlet and to a Panamax conditioner.

Could another component in the chain be adding noise? I had a groundloop issue with my PS3...HDMI can pass on noise...but I fixed that....could an old analog tape deck or some other component cause the popping?

I used Axiom's Coax in line ground loop fixes for my Tivo 3 and cable modem...which is how I got the PS3 to behave.

Look forward to your thoughts. Thanks again.

Ok, tried de-oxit, swapped cables, and unplugged everything from the receiver. Still popping at shut down, turn on, and when I switch some sources and/or decoding mode (e.g. THX games vs PLII vs DD).

Is it possible that the pops are simply coming from the receiver (which I'm using pre-outs on)?

When I hook up a Sunfire out, the pops are way less pronounced, still there, but I attribute that to the newer "soft start" of the Sunfire and the potentially less SPL...as, the Citations I have tried them bridged, biamped, un-bridged...singled...same issue.

10 years later... looks like I have the same issue. Every single input change, surround change, even selecting from an Apple TV to Netflix and all I get is Pop pop pop from my 7.1 Citation. Wired same way as my Citation 22 or if I use a PA4000 no issues. I even tried my HK 990 and it’s fine. As soon as I add the 7.1 into the configuration pop pop pop. 
My set up:

Either Onkyo PRPZ 5100 or ONKYO TXRZ840 as preamp

citation 22 as center
citation 7.1 as LR bridged
PA4000 as rear and height channel 

wiring is all monster cable 
Monster AVS2000

speakers
JBL L7’s and L1’s 
or 
JBL Studio 590 and 580’s

any help would be appreciated. 

I've had the citation amp since new. When it started to age i noticed an on and off pop only.  Have used nad m, 3 different arcam processors. Worse on arcam av9, would pop every input change.  Talked to some people at local store, they thought capacitors were starting to go. Currently use arcam avr850.  I could bi amp with arcam but can't get myself to part with the citation amp 23 years later. Still love the sound and drives the my main speakers effortlessly