Thanks for the reply, lots of guesses but all correct!
I use the PC (not plus.) with AES cable, but have used XLR out to PA / powered speaker also.
My interface needs an upgrade to be honest, it's a M-Audio fastback pro. When I plug helix into this via XLR my signal was fine but obviously I can control the preamps on the M audio unit easier. Studio monitors are Fostex PM1's.
As I want to reamp the signal, I'm using USB which is when it's so loud.
I'l turn the monitors down and keep my volume on helix where it was I think, it's not distorting just stupidly louder. When I upgrade my interface I may find it's the same as the helix, ie the m audio is the problem!