You really need to read your owners manual. You have this hooked up incorrectly. You can tap into the speaker terminal but you need to run that into the HI level input of your sub, not the low level via an RCA input.
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?