Any idea what the resistor goes to? Looks like some regulators on that board? Also some jacks? You've probably checked for a simple problem, but this schematic may be close:
http://music-electronics-forum.com/attachments/23732d1370622849-spider-iii-1508-3012-hd75.pdf
I would think most of those amps blow the power amp and get gutted, so the boards could be good, on the other hand it would depend on whether the seller was parting out amps or came across a pile of junked defective boards, in which case they could ALL be bad.
http://music-electronics-forum.com/attachments/23732d1370622849-spider-iii-1508-3012-hd75.pdf
I would think most of those amps blow the power amp and get gutted, so the boards could be good, on the other hand it would depend on whether the seller was parting out amps or came across a pile of junked defective boards, in which case they could ALL be bad.