Maybe his is a ringer while yours is a dog. But, I'd bet it's the setup (enclosure, vehicle acoustics, wiring, source unit, tuning, crossover points/slopes, soundproofing, etc.) There are a billion variables besides the amp's output.
I'd investigate some of these before possibly wasting money on a new amp.
BTW, the JBL mono sub amps are pretty solid. Check out
www.ikesound.com and
www.bsless.com for some good prices.
