I did some digging around and found the owner's manual for that L121P5 sub and found this:
http://www.xplodsony.com/files/manuals//XS-L121P5_Owners_Manual.pdf
While I see what you're doing by matching the max power of the amp to the max power of the sub, equally important (perhaps even more important) is matching the continuous (RMS) power of the amp/sub combo. See, max power is only ever really achieved in short spurts and peaks, but continuous power is the average load the component outputs/receives.
The sub is rated for a continuous 350W RMS @ 4ohm. The amp you're paring it with is rated at 500W RMS @ 4ohm. So, at first glance, the amp may be just a bit too powerful for the single sub. I wouldn't go as far as to say that the amp will blow out the sub with the extra power, but with 150 extra watts, you'll want to be careful with extended periods of max volume listening.
I just got a Sony XS-L121P5 subwoofer here is a link to the sub http://www.carsound.com/articles/publish/printer_17.shtml
I still need an amp and I was looking at getting this one Sony XM-2002GTR
http://xplodsony.com/xplod/amplifiers/XM-2002GTR
I'm getting it for 100 bucks though, but i've been doing a lot of research and it looks like that amp will be compatible with that subwoofer, I only have one of the subs so I was wondering if this amp will work just fine for this amp? Thanks for the help!

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