If you switch speaker cables and the problem switchs speakers then....it is not the speaker. If the problem doesnt switch then....it is the speaker(unlikely) or one of its cables. Try this first. Put everything back as before.
Then, remove the inputs from the amp, if the problem persists then....it is the amp. If it does not, replace the inputs on the amp and begin removing source inputs from your Preamp. One at a time, then power up and listen. Whenever the problem does stop the last piece disconnected and its cabling are the culprit. If you disconnect all the source inputs and the problem persists then...the culprit is the Preamp, to make sure, disconnect the inputs to the amp again.
A pain in the butt? You bet. But process of elimination is the most sure fire method. Good luck.
Then, remove the inputs from the amp, if the problem persists then....it is the amp. If it does not, replace the inputs on the amp and begin removing source inputs from your Preamp. One at a time, then power up and listen. Whenever the problem does stop the last piece disconnected and its cabling are the culprit. If you disconnect all the source inputs and the problem persists then...the culprit is the Preamp, to make sure, disconnect the inputs to the amp again.
A pain in the butt? You bet. But process of elimination is the most sure fire method. Good luck.