I don't know whether or not these MediaTek/Trendchip devices (DLink DSL-320B-Z1) do support monitored tones. I somehow doubt it, don't know why, but I just imagine they were a low-end product.
The SNRM tends to drop pretty quickly then sometimes stabilises at say 0.8dB. I wonder if that pattern might be an indication of support for monitored tones or lack of it. Perhaps G.DMT or some other kitizens know something about this chipset.
They tend to hold on to sync for a long time these days, but in the past they used to give up and desynchronise after about a week or so. However there is no real pattern even now. It is a long time since I realised that AA was sending them out with the default setting which is bitswap off.
As for error rate, I have not had them running for very long. One of the modems has racked up a lot of HEC errors - about 2% per cell - in a short time. Currently they are all running very fast, with d/s sync rates over 2800 kbps.