I've used both the IRF540 (no suffix other than PBF) and the IRF540A with no problems. The IRF540N was the transistor that caused problems in some amp but those were class AB amps. I don't know if they cause any problems in this amp.
I've never had any reason to measure the resistance between those two legs but it seems too low. Pull the regulator and check the board and regulator to see where the low resistance is.