Where's it failing at...r-card, doubler, or c-card? Not sure how your circuit is designed but get a tester, or if you have test access, start by looping the c-card from the dacs or closest dsx, and stress test the circuit with pattern T1 Daily or 55 Octet. If it passes, loop the doubler and if that passes go for the r-card. This should give you an idea about what section of the span is bad.

Also use the PM data. The Adtran cards pretty much record everything and if you can decipher the info it will give you a clue as to where to look. If it is a model that has the Tscan feature it should have a bad slice detector under the troubleshooting menu... it will give you the distance to the bad splice. Sometimes it is right on, other times you need to give or take a few hundred feet.