[elevateconsulting]

1. ) Actually a QTX brand QA400 model, yes it does have balanced 1/4 and XLR inputs.
2.) Yes, probably the reason for the overall volume loss?
3.) Yes, I have all my gear coming from the same power circuit. I don't have any other outboard gear where my amp is racked. I also tried with the amp mounted in my rack and then removing the screws and leaving it "decoupled" and had no noticeable change in hum volume
4.) so disconnect the shield on the amp end on the balanced 1/4 cables from the Apogee to the amp?
5.) Yes, I think you are right. From what you described in point 2 the "isolator" is probably attenuating the hum at the expense of overall volume.

Thanks for your thoughts Dave, they are mucch appreciated!
I will give disconnecting the shield wire at the amp end a go and see how I get on.