Anyone else here come across this. I have a DR60 and noticed that when I connect it via a USB to my PC, or my laptop, there is a clear humming noise when recording.
If run off batteries or USB power pack it is quiet and records great.
I got really tired of the battery hassles with the 60D. It just doesn't like rechargeable AA's. I got a $30 Anker USB battery, and it just rolls all day and barely uses 20% of its power. My regard for the Tascam shot way up after that.
Little iPhone-style cube chargers work great though.
Biggest issue I have (with the original, don't know about the MKII) is the metering - I can be metering pretty hot and still have very low levels. Finally started cranking the inputs and relying on the -6 for backup.