I think @viridian nailed it.
Do the two balance volume pots change the output? It would be unusual for both the main and the two balance pots to not work.
My Cary SLP-05 volume control does not work
My preamp has been boxed for 3 years and when I hooked it up there is volume to the speakers but the volume control does nothing to raise or lower the volume and the remote does nothing. An ideas?