Distortion, overdrive, and boost pedals are weird with the JVM. On the clean channel, i think they sound like crap. But on OD green or clean/crunch, they push the amp a little harder. I have a 210H and a 205H, so what I do is set up my clean/crunch gain to about the 9:00 position so there's not much gain, then i kick on my Way Huge Angry Troll
to give it a little bit of grit. Then on my OD channel I have the gain at 3:00 to cover all the heavy high gain metal tones. If you have a 4 channell JVM, i wouldn't think you need anything in front of the gain stages. Unless you like the tone coloring the tube screamer adds. Or mabe a fuzz pedal, thats something totally different than distortion or overdrive. I would recomend the Angry Troll, it doesn't color your tone that much, but can give you some nice boost if you use single coil, or split humbuckers. The Angry Troll does something really interesting in front of a crybaby too! Kind of a metallic, grinding, icepick in the side of your skull thing.
I also have a Vox Saturator pedal that sounded horrible with the JVM, but when used on other amps, it was really sweet. It sounds like a hot rodded boss ds1, and it has the "more" button (that's really the only reason I bought it).
As a matter of fact, Joe Satriani doesn't even use his saturator anymore since he left peavy and started using JVM's.