I tried the battery with XLR phantom power disengaged and power supply removed, also tried a couple of different XLR cables and 1/4 cables to rule that out.
I've tried again just now with phantom power, power supply and battery and everything is working just fine using multiple outputs, no idea what the problem was yesterday but I'll send it back for replacement if it happens again.
I'm really happy with it apart from that hiccup, it's miles better than the Ampeg I was using