Loop 5 is line level but then you can't run the 4 cable method totally (like if you want your time-based effects in the loop but other effects pre-input). The regular outputs are not line level. I use the Ebtech line level shifter and it works like a charm. Before that I had all kinds of volume issues...adjust one and the other would go out of whack (or there would be a lot of noise). The Ebtech solved it.
One way or another, the G-System takes a lot of time and effort to get it set up and running properly. I think a lot of folks give up on it pretty quickly, but once you get it up and running, I don't think there's anything else that compares. I haven't had any effects jones for along time, and the big draw for me is being able to save the loops/effects in patches along with the channel change on the MOD50, allowing me to ditch the MIDI footswitch and just use the G-System. Absolutely top notch sound quality too. No "digitized" sound from it like with other multi-effects boxes, and I'm very (overly, probably) sensitive to such things; I ditched all digital effects for about a decade because of it, and the G-System is still the only multi-effects unit I can tolerate, though some individual digital delays, etc, are OK. I'm hearing great things about the AxeFX too, really want to check one of those out, but it wouldn't replace my G-System, that's for sure ("if it ain't broke, don't fix it.").