Problems with a Phase Technologies HV-800 subwoofer

Hi all, I'm new here so please pardon if I put this in the wrong category.

I recently got my hands on a secondhand Phase Technologies HV-800, which I have just recently added to my current setup. it seemed to work fine for a couple of weeks, it had good sound and worked well for a secondhand speaker (there's no telling what the previous owner put it through). The problems started a couple days ago when I powered it on. The moment I powered it on, it began making a very loud humming noise, and when I attempted to play music through my Sony STR-K502P Receiver, it was incredibly loud and boomy, no matter the volume of the receiver. it was rumbling way too much and I powered it off almost instantly for fear of breaking something. 

I have done what little troubleshooting I know how (i.e. changing settings on the receiver, and adjusting the knobs on the sub itself, checking all wiring and whatnot), but I haven't found a solution. looking online bears little results, so I came here for help. Thanks for reading, any help is appreciated.