It sounds like a ground issue with the amp. Disconnect the ground wire from the zed amp and measure the impedance (ohms) between the wire and the ground terminal. Amp off and don't touch the power terminal or wire. What does it read?

Next I would try different rcas. They are cheap and do go bad quite often.

If none of this works go back to the shop and ask them to try another amp in place of the zed. Just hook it up and see if the noise is still there.

I would also check the ground the jl amp but I am overly anal when it comes to electrical.

any resolution on this?

Ive noticed a lot of people with noise issues with zed amps, Ive noticed mine is very sensitive. In my bronco It made no noise, but when I switched to the 300 which is a uni-body Ive been getting alternator whine when the gain is turned up a little bit. Going to try a big bolt and ground everything in one spot, instead of the stock stud that everything is connected to currently.