I am a bit outdated in terms of mode. I guess their model is now here which is being offered for the same price in local shops. Do check and be sure about its authenticity. For a subwoofer I would only recommend that you first get a mono amplifier. Running a sub on a low powered 4ch would be brutal A good powered 2 ch bridged as mono would work as well.
Upto 7k - JBL GTO1214 or Infinity 1250w
Upto 15k - Infinity Kappa Perfect 12VQ
Upto 20k - JBl P1224