I don't have a S3X, but the first voltage test on BH amps is often to test DC voltage after being rectified from AC voltage. If you are supposed to get 100+ volts DC and are seeing less than 1 volt, I'd double check that your meter is set for DC (straight lines) and not AC (wavy line).
I'd also double check that the fuse is inserted in the correct holder position. There are likely 2 positions - the lower one is the correct one; the upper is just a storage space for a spare fuse.
Also: I'm assuming the amp passed all resistance checks?
If none of the above help, I'd post some detailed pics of the wiring.
cheers, Derek