Hi, A newbie Helix user with a Shiva here too!!
I have it running into the rack send at line level which gives me a good Unity level - had to run global EQ a little to get it totally transparent but its there now.
I've come from an FX8 and this thing is so much more flexible - could quite easily do a whole gig (in a covers band) on 1 preset!!!
Snapshots vs amp switching giving me some problems as I can only get it to work by assigning an EXT AMP switch rather than the instant EXT AMP - but that could be me being dull!!
No Ground Hum, however, I am using the old Fractal humbuster cables...