Try swapping tube V13 with V23. If the tube is faulty, CH2 Lead should come back to life and CH1 Lead should sound dead instead.
Please refer here:
http://tubefreak.com/schema.htm#quad
For tube position.
And please make sure you have disconnected your Quad from the wall wart and left like that for a few minutes before you open it up. Inside your preamp there are potentially lethal voltages.