New progress -
Adjusting the polling rate from 1,000 to 125 Khz got rid of the mouse noise. On the negative side, there's a bit more mouse latency but it's worth the noise going away.
Now all that's left is the noise coming from any sort of activity, such as a program loading or closing a window. I'm going to buy new 1/4 TRS Cables, but it'd still be nice if I could eliminate the issue from the source (the pc) somehow.






