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.
sounds like it could be a ground loop I would try lifting the ground on the sub with a cheeter plug and or grounding the sub to the preamp/amp. Maybe try a different cable also. I don't think the driver is the issue.
I just found a number of small tears in the surround of the woofer which are obviously the source of the air leak. I don't know if this is also the source of the buzz, but I guess I'm due to replace this woofer, and am now accepting ideas for a good 8-10" woofer. I say 10" because I am totally open to fabricating a new enclosure for this amplifier. It will still be sealed, but besides that, please recommend a woofer for me that will respond quickly and won't buzz! I have yet to find any Monitor Audio metal woofers for sale, But I'd be open to those or anything of like response. Thanks!
When I connect the same signal cable to the ASW110, the buzz is not an issue, so it's not the signal cable. I suppose it could be the ground on the power cord, but I guess I'd just be surprised as it seems more like a woofer deficiency/mechanical issue (does not seem signal related, but rather output related). I would try your suggestion with the cheeter (cheater?) plug, only I have no idea what you're talking about. I also wouldn't know how to ground my sub to the preamp/amp; are you talking about the power ground or the signal ground? Some clarification might help me a lot. Thanks!
You can repair the surround on the woofer. It is probably still good. New foam surround kits can be bought on the net or Ebay. It's pretty easy to do.

A cheater plug is something you can get at any hardware store for 50 cents. You plug your three prong plug into it. The cheater plug itself only has two plugs.
So the purpose of the cheater plug is to remove the ground loop noise from the sub by not grounding it? I actually own a couple of cheater plugs, just never heard them called that. I'll give that a whirl, thanks! I just have to find them...
The cheater plug might help if there's a hum caused by a ground loop problem. The 'farting' sound is definitely caused by the leaks in the foam surround. As Warnerwh suggested, there are kits to repair them. But depending where the rips are located, it might be better to purchase a new driver to ensure a proper seal for the best performance.
The rips are located at the apex of the curvature of the surround, at where a few of the bolts attach the woofer to the cabinet. I guess this just must be a point of stress for the surround, but it is foam, and decayed so much that I think I just need to buy a new surround. I will see if I can find one somewhere, it looks pretty standard issue. I'd hate to buy another driver, I'd rather just sell the amp and use that money to buy a whole new subwoofer, I see some cool ones such as the Aperion S8-APR go for not too much and would certainly be better than what I've got (or could have, even with a new woofer). I digress, I will look into replacing the surround.