Good News!
This headphone amplifier has been REPAIRED and is now working perfectly.
Here's what happened:
I have been taking parallel measurements between the "good" and "bad" channels since the beginning.
I took voltage measurements, resistance measurements, tested ALL capacitors, and tested ALL transistors.
But someone in my original thread (I can't remember who offhand -- sorry) did mention checking the circuit board traces.
"No need," I thought. "The circuit board and all solder joints look fine to me."
But in a fit of desperation, I nonetheless decided to whip out a tester I hadn't used yet: my magnifying glass.
That's when I found it -- a *microscopic* crack in the trace between the emitter of Q208 and its adjoining 3.3 ohm resistor. The crack was so incredibly tiny that I easily overlooked it--repeatedly.
I took one of my jumper leads and clipped one end of it to Q208's emitter and the other end of it to the 3.3 ohm resistor.
I then took voltage measurements. EVERYTHING WAS NOW ON SPEC:
Q205's collector now measures +7.09V, and Q206 now has the following measurements: B: +6.51V C: +7.08V E: +5.84V
I was able to bridge the crack with a well-placed solder trail, and the headphone amp now sounds great.
A BIG "THANK YOU" TO EVERYONE WHO HELPED ME OUT! I OWE YOU BIG TIME.