Hum in left channel - Parasound Z

I'm using a parasound Z amp and a z tuner at work. I do not have a preamplifier, so I have been using the gains on the back of the z amp. It works fine, but it's a pain to have to reach to the back side of the amp to turn volume up or down. Plus, matching levels by channel is difficult.

I obtained a 10k stepped attenuator that I wanted to use as a temporary volume control. It's stereo as I didn't want to adjust each channel independently.

I tried the attenuator at home last night with a different system. It worked perfectly. I brought it to work this morning, connected everything, and it worked great. However, about 30 seconds in, I developed a hum in the left channel. Now, I can't seem to make it go away. It's a 60 Hz hum. I've swapped speakers, and determined it is the amplifier. I've also ruled out the source.

My ultimate question is, is it likely that I damaged the left channel of the amplifier with the 10k attenuator? If so, is there something obvious that can be replaced in the amplifier to make the hum go away? The left channel operates correctly, just has a hum - which is annoying.

Thanks for any input you can provide.
I just swapped the inputs going into the amplifier and it stays in the same speaker which (I think) confirms it is the amplifier.

When I swapped speaker cables, the hum also moved.
do last test and connect pc directly to amp using pc soundcard volume control. if that fails, i hopw you still have a warranty on your zamp. also your amp could've been a victim of a surge. digital amps need to be protected because of high volnurability of the digital control board.
The amplifier is out of warranty. Please correct me if I'm wrong, but I do not believe this is a digital amp. I believe it is an AB amp.
There are a couple of possibilities I can think of.

One possibility is the hum has always been there but you didn't notice it
because the volume control on the amp was turned down. Now you have
turned the volume control to the max to use an attenuator and the hum
becomes noticeable.

Another possibility is that the hum is coming from an appliance you didn't
have before, perhaps a new light with dimmer, a heater, a fan, etc...

I have run into both situations before. It is worthwhile to check out.
strange. for some reason i thought that zamp is digital, but you may be right. what speakers you're using?
Thank you for all the feedback.

I'm using a custom set of speakers design by Clearwarve. The speakers have performed without issue, and I swapped speaker cables to cause the hum to move.

It is only in one channel of the amplifier. Not both. I plugged the amplifier into a different power circuit and the issue is still there. I have played this setup with decent volume before and have never had issue.

It was only after I ran the setup with the attenuator in place that the hum developed and now won't go away.

Any reason to believe that there was some DC offset being carried on the positive side that may have done some damage?
DC offset over tolerance is usually when DC caps getting old and drain reverse voltage. Zamp shouldn't be that old though.
A couple other notes. I used a cheater plug to lift the ground, no change. The volume of the hum is not impacted to the gain on the amplifier. In other words, I turn it up, the music gets louder, the hum stays the same.

Thinking of dropping by an electronics repair shop on the way home tonight.
That attenuator seems to have done something, or exposed some issue. It's worth phoning Parasound directly, their service is amazing. Call 415-397-7100. They may even give you the owner, Richard Schram, to talk to. Many have reported what a knowledgeable person he is.
I called Parasound and talked directly with Richard Scrham. Gotta love a president that answers the phone and talks directly with customers.
I have a same problem with my amp which is in service center now and it is AB amp(ML no532).
According to the service center it is most likely bad capacitor and it is easy fix for them.
I had a hum in my system caused by the cable coming into the house from Comcast. When I disconnected the cable the hum went away.
He feels it was unlikely that the attenuator did any damage, since it is strictly passive. When I told him I would likely take it to get fixed locally, he offered to send the schematic to the person doing the repair work.