Bazaar buzz

You shouldn't ever unplug anything while the amp is on, this lead to blow ups.
Do what I said with shorting rca plugs. If it's their while both inputs are shorted, it needs a tech to look at it.

Cheers George  

I don’t have any shorting caps. All I know is:

no IC’s connected = zero buzz/hum

two IC connected to amp but NOT preamp (just hanging loose) = buzz/hum - loud in dB. Also if I move an IC around buzz/hum changes a bit in intensity. 

two IC connected to amp and preamp = buzz/hum - lower in dB but definitely there

one IC connected to amp and preamp, but the other not connected at all = Zero buzz/hum. 

Get some shorting rca’s plugs (not those stupid rca caps) it’s the only way "you" are going to find out if it needs a tech to look at it. Both channels should be quite when these are in. If not see a tech.

https://www.ebay.com.au/itm/Onkyo-TX-DS989-Stereo-Receiver-Parts-Original-RCA-Shorting-Plug-Noise-Terminator/173848128831?hash=item287a27ed3f:g:x74AAOSwkNFclo7J&frcectupt=true

https://www.ebay.com.au/itm/RCA-Shorting-Plug-for-Vintage-Fender-Pro-Reverb-Amp-No-Need-for-a-Footswitch/174113224229?hash=item2889f4f625:g:5C8AAOSw5VFWMAHP&frcectupt=true

Just an update I just used a standalone transformer with the amp, and all the buzz and hum appears to be gone!  I say “gone”, unless you put your ear right up to the woofer and then you can hear the slightest, slightest hum/buzz. Certainly not audible from the seating position.  If you put your ear to the tweeter or mid range you just hear your normal hiss, with no other artifacts. 
I’m assuming this points to dirty AC, correct?

🤷‍♂️🤷‍♂️