Source of woofer buzz?

So I bought a M&K VX7 on ebay, and it didn't work. After hearing from M&K that the amp was irreparable, I bought a 100W Parts Express sub plate amp, and plugged it in the existing enclosure w/ M&K subwoofer, and it works. I have no idea how good or bad in comparison to an M&K, probably worse, but it sounds decent to me.
I have it currently running side-by-side a Monitor Audio ASW110 for comparison, and both sound similar for music listening, but the ASW110 sounds a touch smoother, and is capable of much more volume, especially at lower frequencies. This is where the problem comes up. I think the M&K would be capable of more volume, only when I begin to turn it up, a funny vibration/buzz comes from the woofer cone that does not sound musical at all, but rather mechanical (kinda like a fart actually). I do not know how to make this sound go away, but I do notice that it appears when the crossover is set higher even with the gain lower (i.e. higher bass frequencies bring it out more). It is amplified when the gain is raised, not so much when the volume of the system is turned up (when I have the system at low volume, I can max the gain out, and the distortion is present at a lower volume but at equal distortion (sounds really crappy), and when I turn up the volume, the distortion just gets louder along with the music, which is when it becomes unbearable).
My only guesses are that the woofer isn't made to put out much volume, and that I'm just overpowering it (thought it's definitely not mechanically bottoming out), that the woofer is simply old and worn, or that the enclosure isn't perfectly sealed, and this is causing some looseness (for lack of a better word) due to the lack of acoustic suspension that this woofer was undoubtedly designed for.
So my options are as follows. I was thinking that I could find another 8" driver to put in place of the M&K and see if it sounds better (provided I can find one that has the proper specifications for my application), but first maybe just ensuring the proper seal of the cabinet (my car sub is meticulously sealed, so I know what sealed feels like when you press on the cone, and when I press on the M&K, it moves as if there's a small leak somewhere).
What I'd like to know is if anyone has any suggestions as to what may be causing this deficiency that prevents me from getting my full meal deal of bass. Any thoughts and suggestions are appreciated.
Oh, and for the record, I don't notice it during movies, only when I get close to the woofer during music playback. And please don't blackball me for modding an M&K, I'm poor and it was economical.