If you followed the story neatly you would notice that (red point X) referred to Normal ch.
We will repeat the test procedure for Vibrato ch.
Temporarily connect test wire from .001uF with anode V2 (pin 1)
Turn on the guitar in vibrato ch and try the sound.
If the sound is a loud temporarily connect test wire to the anode V2 (pin 6) and try with a guitar.
If the sound is loud and clear and the volume and volume controls works, temporarily connect test wire to the anode V4 (pin 6) and try with a guitar.
If the sound is loud and clear and tone and volume controls work, check .1uF (pin 6 V4), network resistor 220k (blue point Y)
If the sound is still ugly:
- check the anode and cathode voltages on tube V2 and V4
- cleans from oxide ground connection input jacks on vibrato ch to the chassis
- check tube sockets V2 and V4 for cold solder joints