I just purchased GPO and JABB and have been playing around with it in Finale 2005. I have a very large score orchestral score using GPO and JABB libraries (around 44 channels).
It sounds great when parts of the orchestra are playing, but during full tuttis when its quite loud, I'm getting lots of clipping and a little stuttering.
My setup is an AMD Athlon 64 3200+ running at 2.04 GHz with 2 GB of RAM. I'm using my MBox's ASIO drivers for playback with Finale 2005 through the Garritan Studio. It's better when I turn off Ambience for reverb, but it still has problems during the full tutti section.
To make sure that clipping is not occurring in the louder sections, lower the overall levels. If this doesn't eliminate the distortion then the problem is probably computer resources. The Kontakt player is very demanding on the CPU and the most important spec is raw clock speed. Really large instrument counts do better with clock speeds in excess of 3ghz. You could try increasing the buffer values for your sound card and host software. Especially during playback (where latency isn't an issue) this can often get better mileage from a CPU.
I created many of the demos on a 2.4ghz machine and, as I just explained in another thread, it was not capable of doing a full big band arrangement without either using mostly Lite instruments or "freezing" about half of the tracks in Sonar. "Freezing" is essentially converting virtual tracks to audio tracks which placed far lower demands on the CPU.
Now, some people here are getting successful performance with rather high instrument counts (with clock rates similar to yours) but there are many variables, not the least of which is the number and types of instruments you are trying to use simultaneously.