i re-measured the chains.

my SRAM 1070 chain is stretched about 1/16 of an inch. that'll get replaced.
my KMC X10SL - the really noisy one - actually appears to be in good shape. Hard to believe since i think i have done harder riding on this one this year.

So, I'm going to try an experiment:
I took the KMC chain off the bike and degreased the holy hell out of it again.
then i rinsed it thoroughly and let it dry.
then i sprayed it again with a bunch of degreaser and repeated the process.
then i put it in my dishwasher and let it go through a cycle.
it's currently sparkling clean and drying out.
if there's any dirt still in it, it's not coming out.

i'm going to give it another shot with a few coats of new lube, and we'll see what happens. hopefully i can learn something about this process.