Jolida 302B low frequency hum

I recently purchased a used Jolida 302B here on Audiogon. I'm experiencing a low frequency hum and am curious as to what the source of this might be. There doesn't seem to be anything external to the amp that's causing it. I haven't biased the tubes yet, might this be the problem? Any advice/suggestions would be appreciated.
Actually, I've determined that if I only connect one source to the Jolida there's no hum. However, if I connect more than one source there's significant hum. Is this a ground fault? Any "easy" fixes here?
It's a ground loop. Type that into the search engine here and you will find a lot of information.