Watching the repair video, at 13:25 he states that the resistors have to be have well matched for the circuit to work.

I posit that you could use 10% resistors, and it would work just as well. The individual gains don't have to be exactly the same, since they're all being summed anyway.

Oh, and those plastic stand-offs really need to go.

This reminded me of an old advertisement which compared a component stereo amplifiers specifications to that of a piece of wire. I spend almost an hour looking with google, etc., and I can't find it. 8(