So, Dave...

This topic seems to have drifted off of the subject of helping you
troubleshoot your amp and onto desoldering techniques and tools...(not
that that isn't great content).

So have you fixed it?

If not, what's this 10-year-old mixer IC you mention? Do you have a
pinout of it? How do the DC voltages on all the pins of the output IC
compare? If you have a meter-based meter (was that redundant?), can
you see any DC fluctuation on the input or output pins of the output
IC's when you spin the volume control? Check both channels and compare
them.

I've seen speaker "thumping" occur when DC was getting to an input it
shouldn't be.