I would like to use my macbook pro and FF400 for some live work, but find that it is very noisy i.e. induced hum and frying noises dependent on whether I am touching the strings or not. The same guitar and lead is clean as a whistle straight into my Fender Delux.  Is there some impedance mis-match? Should I be using a preamp into the fireface? I would be thankful for any clues.