Thanks all. After some experimentation it seems it may not be the IC after all - or the grounding, for that matter. The ICs (with its current routing maintained) when connected directly to the line level source which normally goes into the MC preamp results in no hum coming from the speakers. Since all my sources are connected to the same power conditioner I figured it also isn't a grounding problem, but for kicks I lifted the ground on the MC preamp - no difference. Since my MC preamp has both analog and digital inputs I tried one of the digitals for this source - hum went away.
So if this source's analog out produces no hum when connected directly to the speakers, and it does when connected to the MC preamp's analog (but not digital) inputs - and it doesn't appear to be a ground problem - I've decided to contact the manufacturer and see what they have to say.
Reubent, I did also try your suggestions and there was no change.