The fact that they're fixed seems contradictory to the manual, which says, under the Zone 2 section for "adjusting the volume":

"Turn MASTER VOLUME after pressing ZONE2 SELECT on the main unit to adjust the sound volume."

The order of that information is backwards and sounds like the result of a translation. How about:

"To adjust ZONE 2 volume on the main unit, press ZONE2 SELECT and then turn MASTER VOLUME."

At any rate, it further states that Zone 2 volume's level can be saved to a Quick Select button, but nowhere does it state whether such control is limited to Zone 2 via speaker output, or also works for Zone 2 via line level output. It's just presented generically, following the line level hookup instructions.