From the schematic I have, the 500K resistors are from the inputs to ground, so would be in parallel with the volume pot. See if that value changes as you move the pot from 0 volume to full.
That said, measuring resistors in circuit will give you sometimes strange results as you don't know the exact interactions with connected components. Also, ohming out resistors for what is suspected to be a leakage problem won't really tell you anything.
Can you post detailed pics of the amp boards? Are the resistors lifted off of the board, or installed right on the surface? And what is the origin of this KGSSHV? i.e. who built it?