GT-8 user here. Like it alot, but not too fussed on the 4 cable method - persisted with this during live work, cause I had forked out for the x-tra cables, but finally gave in and sent all through the front end of the amp. May be different with a different amp though, as my master volume on the amp is placed before the fx loop, so fx return was always running the power amp at full throttle, the overall volume then controlled by GT-8 output level. Not a very good sound level to noise ratio.
Really only practical use of a multi like this, still using the high gain channels and clean channels of your amp, is to get midi control - otherwise you still have to change amp channel and fx patch seperately.