Different comps and limiters are designed to act differently and perform different specific functions. A limiter built into an amp may be anything from a heavy squashing compressor at the input stage, to a nearly-transparent peak-stopper at the output stage; and it may sound good, bad, or indifferent, partly depending on personal tastes, and depending on how loud you play, through what cabs.
You might want to find threads specifically about the GBE 600 to see other owners' opinions about the limiter. It helps if they mention the types of gigs they're playing, and the types of cabs they're using.
Genz is also really good about answering questions from customers, so you might want to PM agedhorse or hasbeen (Genz guys here) for an explanation of what the GBE 600's limiter was designed to do. Then compare that against what you were wanting a compressor to do for you.
