On the mid range branding, I don’t know. I just pulled that schematic from this site. In terms of the resistors, I heard of situations where the resistors appeared fine and when removed were burned and cracked on the bottom. The Dahlquist included resistors are just basic and not super high-quality.?
I would take a close look at all the solder joints with strong lighting and a magnifier if necessary. Look for a smooth shiny appearance.
Swapping midranges will elliminate that as an issue.
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Charlie Conger