Just an update for anyone who may be caught out with this like I was - it turns out the hiss happens when my recording computer, a Leader laptop, is plugged in to its external power supply.
Unfortunately, as it sucks quite a lot of power that’s going to be most of the time, but at least I know what’s causing it and will be able to turn off the charger when mixing.
The distance of the charger or the laptop or the orientation of them with respect to the monitor amp doesn’t seem to make a difference, which I suppose means the signal must be travelling into the amp via the CI2+ output cables, so I may be stuck with noisy monitors. It’s not that bad with respect to overall signal level, just annoying.
I’ll try the balanced leads when they get here, might make a difference.