J-Rob: Sorry for the tardy reply, but it's been some time since I looked back at this.
Ya, I've got their 303 Fabric Cleaner too. It doesn't seem to do anything most similar cleaners won't?! I tried it on a nasty old piece of marine canvas with lotsa acid rain stains, etc. It brighten it a little, but no miracles.
I've got one powder cleaner that works great: IOSSO Mold & Mildew Stain Remover. This works so well that my 2001 Ford has never been waxed (since Aug2000 and 94,000mi) but is shiny as hell the minute I wash it. It seems to clean anything. The only downside is that my hands become slippery as hell when wet for a few hours afterwards. I've been told by a guy who washes planes for a living that this is the sign of high PH - whatever that may mean.....