sounds very odd..... but unfortuantly without actualy looken at it first hand its realy hard to diagnos the problem

your ohm load tests are just find... they are always alittle lower.... no problem thier... now with the sub wired to the amp touch the tester to the amps terminals and check your ohm load thier... maybe ya wired them wrong inside the box... BUT if that reads 2ohms then your wiring isnt a problem..

next test your RCAs.... pull one RCA off the amp.. you should still have output BUT not as much... just maybe a rca came off the back of your headunit or they got pinched shorting them out... try them both that way.. ya should have more output with both pluged in

and if those are both good then id just play with the gain control alittle more.....
ive had several amps that just didnt work very ywell with some subs with the gains set where they should.... BUT just didnt.... i adjusted them up as much as i could that still sounded good! just listen to the sub for awhile and feel the cone.. if it gets to hot on the center of the sub, dust cap... then turn it down or youll burn out the voice coils....
i never set the amps where it says to set them.... i adjust by total ear.... just tinker with it forawhile.
good luck! BUT if ya ditch the box just stisck with MDF and carpet..... if you can do that fiberglass im sure you can build a nice mdf box.....