Been enjoying this Ampeg tube preamp for a few months now. One quirk: it puts out a loud hum when there is no instrument cable in either input jack. (Never had another head or preamp do this.) Is this normal for certain amps? Or is it some kind of minor malfunction? The unit seems to be fine, otherwise. Expert speculation and/or insights would be most welcome. (Please pardon my ignorance - this is my first excursion in to SVT-land.) TIA!