It has a separate volume control on this unit for zone2. Fixed or variable doesn't matter if you are using the receiver to power zone 2. As Duffinator said, it must be analog for it to work, so I don't understand "sporadically" either. If you changed zone 2 to a source that uses a digital connection, you will get no sound.
601 is 6.1 only, and when you use the receiver to power zone 2, the main HT is limited to stereo as well.
In either case, one question that needs to be answered is whether you are using a separate amp/amplified speakers or are you using the receiver to power zone 2?