As will become immediately obvious, I know almost nothing about this.

Anyway, my girlfriend has an older MTX amp (looks to me like it might be an old version of the X Thunder series) She seems to think it's 300 watts.

Anyway, the sub she was given was blown out and now she's got to find a new one.

She listens to mostly Rock type stuff so she's more interested in something that's got a more tight focused and clear that will add to the music instead of something massively loud and boomy that you'll hear from 4 blocks away.

 

Any chance you can give us the model number of that amplifier? It'd make helping you out much easier.

If it is 300W, I'd suggest a 10" Alpine TypeS, in a small sealed box.

http://www.bestbuy.ca/catalog/proddetail.asp?logon=&langid=EN&sku_id=0926INGFS10 098281&catid=20118 <<< Link to BestBuy sale on the TypeE, which is Alpine's entry level gear. Its a very good brand. If her amp turns out to be 300W, then it'll be a decent match with this sub. For the price, you can't go wrong. Best to have a sealed enclosure made to spec, to get the most out of the setup.