A rough frequency? What the hell is that?
Both Zapco and JL are great products, but to be honest, you could probably go with something cheaper and achieve the same results. However, I'm not telling you how to spend your money, so you wouldn't be disappointed at all with JL or Zapco. One thing to consider, I guess, is that JL has a very regulated output while Zapco does not. Meaning the 500/1 will put out exactly (or close to) its rated power all the time while a Zapco will produce fewer watts at a lower input voltage but more watts at a higher input voltage. Neither way is really any worse than the other, just something to consider.