actually i had a very low noise floor with the g system BUT i wanted the isp in the rack just in case for poorly wired venues, i play proffesionally in many outlets from country road player to studio player. im currently touring in place of Brad Noah in the christian metal band Disciple, and i just need the security blanket of the isp and it has come into effect more than once, especially on outside festivals with a ton of lights onstage. To answer your questions: you might be running into a ground problem with the g system. i have instrument cables connecting everything as normal but each time the signal passes to a new piece of gear in the chain i lift one end of the ground on the cable. Buck from isp advised this, it seems alot of the big boys like Lukather use this trick in their elaborate rigs to reduce hum. i use the decimator with the extra output, if you contact isp they will gladly walk you thru the wiring of this rig, their customer support is top notch. also for the best sound of the g system turn the boost function on and set the levels to 10db for every patch, as their "boost" is nothing more than a signal loss and when you turn the boost on it allows the full signal to pass thru. Resulting in the purest tone of the amp. i run everything in the seriel loop, Peter outdid himself on the quality of the loops. Also if you need the boost use the second master on the amp. with herberts two master volumes their is no need for another boost anyway as it is already on the amp. hit me up if you need anything else.
best of luck
Andy Wood