Which amp will give me the best response from my sony XS-L121P5 sub.?

I have an Alpine 500.1 amp, and it sounds great below 50hz. The only thing is, I wasn't able to use the crossover function on the amp, I had set the amp to 'full range', then add a line filter. Sounds much better.

I disagree with Leo.

There's not a sub amp out there that (or any amp for that matter) won't produce signal at 20hz.

The issue isn't the amp - it's the sub and/or the enclosure. Most basic single 12" subs are going to struggle to get near 30 hz with any authority. However, in the right enclosure that particular sub should give you usable output at least to the mid 30's. If your box is sealed 30hz isn't realistic. It needs to be ported and correctly tuned to get the best low frequency performance.

I found a thorough review discussing the sub including some good information on the enclosure: http://www.carsound.com/articles/publish/printer_1...

As for an amp - you just need a decent 2 channel that bridges to around 400w rms.

Any of these (among others) would work well:

http://www.sonicelectronix.com/item_22698_Hifonics...

http://www.sonicelectronix.com/item_37128_PowerBas...

http://www.sonicelectronix.com/item_37743_Pioneer-...

http://www.sonicelectronix.com/item_26850_JL-Audio...

http://www.sonicelectronix.com/item_27731_Massive-...

The problem here is that amps will play 30Hz but the volume in which it plays it is much less than the higher frequencies (Attenuation). Therefore a lot of manufacturers will class their amps at 50Hz+. At least that is realistic. Instead of somebody claiming 30Hz that you can bearly hear.

Although your sub is rated at 30Hz because it can handle it. Meaning it can possibly just be heard at 30Hz.