If your switching unit is a passive preamp, and if you are handy with a soldering iron, try bypassing the volume pot.
Hum could also be simply poor earthing arrangement.
You can also short any unused inputs with a "shorting plug" - put a resistor across the +- inside an RCA connector - this is better for eliminating noise intrusion than the "audiophile" shorting RCA's sold that are simply a dead short.