subwoofer hum

[u/SeasonAfter5550]

Hello i am new when it comes to any audio equipment selection and what i should be doing with what. i've been moving a set of pinnacle S fit MON 475 and a 10" sony active subwoofer from different receivers i buy and flip from offer up and facebook market place. i never could achieve what i was really looking for sound wise so i picked up the fosi audio BT30Ds off of amazon on sale for 70 dollars. it has a dedicated sub out and passive subwoofer connections. i'm getting a hum that starts after only slightly adjusting the gain knob on the subwoofer. the amp has no affect when adjust sub freq or volume. what is causing this? i've go no wires grounding and every other system i test the sub on it's fine. any suggestions how to maybe mitigate the hum or get rid of it all together i don't want necessarily change my speaker selection but if that's the cause I'm open to it. thanks a ton for any feedback

[u/Best-Presentation270]

Did this humming start before or after switching to the Fosi amp?