Sven,
from my limited exposure to the GA preamp (Beagle had one at our shoot out), it follows the neve model w/ an input and output knob, so you can saturate a signal. I wasn't impressed w/ the saturation so much. The Tone Beast sounded better, all the way up thro distortion. The guitarist I was testing w/ (who owns a pro studio here in Dallas) thought it might be an answer to a "crappy guitarist w/ crappy tone on a crappy amp." He could add it w/ the Tone Beast, and the DI'ed acoustic we shredded did sound pretty electric.
Other than that, most of the setting changes were pretty subtle, tho note that I didn't have a different op-amp in this proto model I was testing. It had the same op amp as in the WA12 , or an api 2520, which goes in the api 512. Production models have a cleaner op amp as a choice, and Bryce (the Warm Audio guy) said it sounded a lot differnet.
So, first off it is a good preamp w/ mic and DI on front, as well as a mic and line input on the back. Handy, esp. for the home studio. But, on the right signals, you can get all kinds of different textures out of the signal. Again subtle, but I could hear it at home as well as in a tuned studio.
It is worth the extra $150 (compared to the WA12) if you like to spend time on that last few percent of sound. When we first used it we did voice on a 421 from the drum room. There was no dicernable difference on any setting. After we turned up the in/out knobs and switched to gutiars, the settings became audible. It takes a temperment, ear and monitoring system as well as time to take advantage of all the settings. I can't see using a Tone Beast on every drum mic, unless I was getting paid by the hour. But lead guitar, rough up some synths and tweak a loud vocals (on a proper mic), yea, it should be worth the time. I've only had it a couple of weeks, but enough to know it is useful.