OK - if you've tested from top_of_board component leads to the actual speaker lugs (not the quick connect), then you've pretty much verified that the solder junctions are good. The only one you haven't tested would be the outside lead of CAP HI (adjacent to INP+). If that junction ain't bad, you may have a bad component - most likely CAP HI (the 2.2uF poly). Testing that would require a capacitance meter & popping a lead loose from the board.
Edit: Oops - 4.2 ohms? Would've expected something closer to 3 ohms, Try measuring the same points on your good board. If they don't match, you may want to double check the solder joints on the 16 ohm resistor.
Y'know, the last time someone reported problems with a DIYSG crossover circuit board, it turned out that they'd accidentally switched woofer : tweeter connections in just one of their speakers. Kinda comic.
Last edited by scotthal; Today at 11:55 AM.
Reason: More info