Greetings all,
I bought an AudioSource AW4 12" powered sub at my local Goodwill store for $30.00. It emits a low hum when plugged in and not in use. Is this normal for a powered sub? It is magnetically shielded too.

no it's not normal but I've never owned a $30 sub. Try disconnecting the cable between the receiver and sub and the power it up and see if the humm is still there, it could be the cable. If it's still there try using a different power outlet.

Thanx for your response Jakeman,
BTW, I have an AudioSource SW 4 subwoofer; not an AW4. I've tried all your suggestions and I still get the hum. I don't notice while listening to music and the hum doesn't increase when I turn up the volume. I just wanted to know if it was normal for a powered sub. Could it be my system is too old? It's about 20 years old. Here are the vitals:

NAD 3020B integrated amp (rated at 35w at 8ohms)
NAD 4125 tuner
NAD 6155 cassette deck (just gave up the ghost)
SONY CDP-215 single disk CD player
Technics SLQ-202 turntable (with cheapo Audio Technia needle. Where can you find phono cartridges nowadays?)
Polk Audio R30 floor standing speakers (recent purchase after years of just using headphones)