Help trouble shooting sub hum

I have my subs run off my speakers using a banana to rca cable off the bi wire posts on the speaker. Funny thing is that if I turn my amp off the subs hum and it gets increasingly louder until I turn them off. in a quest to better dial in the sub volume I added a passive preamp in between the speaker and sub so the sub’s volume can be set at 12 o’clock instead of 4 clicks above zero. Then I want to use the attenuator to fine dial the sub in.

it works as expected except that wnow with the sub coming up much higher there a constant hum barely audible from the listening position.  Since it existed before the passive went in it’s obviously something else. But what?