Don't forget to ask him to ring for the recalc when he has cleared the fault.
AC balance should be high, above 55db, but the higher the better, maybe you misheard him ?
He has you running in the data extension kit yourself ? If he's that good a salesman........
Small price to pay if he finds and clears the fault. Check out the sync rate, SNR, error count on his tester when finished.
400m, you should very nearly see full sync, unless the D-side is aluminium.