I haven’t had my subwoofer hooked up for quite awhile. When I hooked it all up today there was a distinct hum when the power was on. I removed the chassis ground screw but the hum persisted!!! Help!

Read these two good links:

https://www.lifewire.com/troubleshoot-subwoofer-hum-3135140

https://blog.teufelaudio.com/how-to-stop-your-subwoofer-humming/

Thanks Mojo! Looks like I’m on the hunt for an isolation transformer!!

Subwoofer Isolation Transformer/Ground Loop Hum Eliminator, Blue Jeans Cable Brand, Made in USA https://a.co/d/4TxgvMb

You may find the bass distorts when using an isolator due to the transformer saturating. If so, change your gain structure by turning down the volume of the sub output of the receiver and turning up the volume of the 400.

You may not have a problem at all if the transformer is designed to saturate at some voltage higher than the sub out (say 10 Volts as most receivers never go above 7 Volts on the sub out).