Bose companion 5 sound issues with Logic Pro X

I am playing electric guitar into Logic Pro X 10.0 on my late 2013 27" iMac running Mavericks 10.9.1 via an Apogee Jam - audio output is via my Bose companion 5 speakers connected via USB (latest drivers installed). Apogee Jam is set to lowest input settings and guitar is not on full volume either and so I know I'm not overloading the system.

Everything works fine and sounds great and then I get a crackling distortion noise after a few minutes. If I change the audio output to internal speakers all is fine again. I then switch back to the Bose speakers and it will sound great for a little while and then distort again. There is no pattern as to what triggers the distortion.

I've tried tweeking all of the settings I can think of but can't see to cure the problem - any suggestions as to what the cause might be?

Any help would be gratefully received.