Although you can use DMTtool to over-ride the target SNRM, my own observations are that in a lot of cases the maximum over-ride figure is 3dB - this may very likely depend on how the DSLAM is configured, but its not uncommon for you to be only able to tweak down 1 stage.
So therefore if your target SNR is set at say 6dB, then dont be too surprised if the maximum you can tweak it to is say around 3dB.