Update - it appears that tsuffing a GLI (Ground Loop Isolator) into the chain would [probably have a detriemntal effect on SQ, unless you spend a oittle cash (I found some american ones for around $150, muhc better than ther £6 affair from Amazon perhaps)
However, and this is where reading the owners manual properly helps, the DASCmagic Plus has a Ground/Lift switch on the rear panel above the USB input. I haven't tried it yet, as am still away for NYE celebrations, but will try this out tomorrow and report back.
Am also going to use some xlr connectors to the amp, and upgrade the USB cable.
This hum/interference loop is very common from what I found when googling for it, but 99% of the answers are for Djs and involve using these cheap GLIs. All of the amazon reviews, where the reviewer states how it is being used, are all in between the analogues output of a PC and the amp, none used with an external DAC.