I've never heard of anyone disabling Annex M in the modem, as that doesn't disable it at the DSLAM. It may cause the DSLAM to train down to proper ADSL2+, but maybe the modem merely loses the extra upstream bins and they don't get re-allocated to the downstream.
At below 16Mbps Annex A downstream changing to Annex M (which has to be enabled by the ISP) will lose ~2.5Mbps downstream wuth a tiny increase in upstream. At a guess, a 15Mbps Annex A would not give any upstream increase on Annex M.
My broadband basic info/help site - www.robertos.me.uk
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Connection - Plusnet
UnLim Fibre (FTTC). Sync ~ 56.6/14.1Mbps @ 600m. - BQM
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- Jean-Antoine Petit-Senn.
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Edited by RobertoS (Thu 21-Aug-14 22:41:18)