I take it you work within ALU ?
If I were in your position, I would try to activate AMR + DL DTX on few cells only, then AMR + UL DTX on few cells, and then AMR + DL DTX + UL DTX. The problem might be already fixed in your release ?
For each trial, wait only 2 or 3 hours. If there is a degradation, you should quickly see it in NPO : high HO QUAL and high HO INTERFERENCE.
To decrease the Call Drop w/ AMR, have you tried tuning the RadioLink TimeOut AMR and radiolinktimeout AMR_BS ? Also, you can change the thresholds for AMR codec change and channel change, along with the averaging window.