I hate chasing noise in a system. There’s so many causes, and it’s usually the one you think is least likely or you’ve initially ruled out.
Just to clarify, with your previous computer you didn’t have this issue. You also state that you hear the noise from the subwoofers through the Crown amp.
What diagnostic equipment do you have? I’m sure at least Multimeter. Do you have an outlet tester? An Oscilloscope would be great, but not necessary, and if you aren’t up to using one properly with lines voltage (all the safety precautions for not only you, but the scope as well), better to not worry about it. Do you have any ground lift adapters, aka 2 prong to 3 prong plugs (extremely inexpensive for a 2-pack at just about any store)?
A video showing exactly how things are hooked up and a demonstration of the sound would help as well.
You have done one of the biggest troubleshooting steps already. You’ve isolated it to impressed noise on the common power line branch. One last thing on that particular line of reasoning would be to plug the computer back into the monoprice strip and then use the extension cord to plug into another (third) outlet on the same circuit. If the noise returns, then it’s hard confirmed to be an issue with all connected to the same circuit.
Next thing I would do is try using the ground lift adapters on the computer first, then the Crown second.
Another question is, how is the Crown ultimately connected to the AVR? The interference may be introduced there.