Having found your router info pages very useful on a few occasions, thought that might be a sensible place for some new info on the D-Link DSL-320B to go (even though its a just MODEM).
Is this the right place to ask??
I've worked out the CLI commands to adjust target noise margin, and save the settings in the config file which others may find useful.
Specifically, the instructions are for setting operation on BT20CN.
The BT21CN target noise margin(s) can also be adjusted, though control via line setup is maybe better.