i just thought of this:
take your headunit out, and disconnect the RCA cables going to the sub amp from the headunit. turn the radio on and turn it up. if it sounds fine, then its the radio station. if you are still getting a shitty sound, then make sure that the antenna is plugged in all the way, peel back a little of the wire in various places to make sure that the antenna wire isnt corroded or anything (sounds stupid, but u wouldnt believe the shit i’ve seen :bloated: ), make sure that there are no places where the bare antenna wire could be exposed, make sure that no power wires are touching the antenna wire or connector, and if you are feeling really ambitious go get another head unit and hook it up and see if u have the same problem.
Also, how is your Sirius hookd up? is it through an FM modulaotr ro direct connect to the headunit if its FM mod, there may be an issue with that.