One thing you could do to help SNR is to remove the ring wire from all the sockets in the house, provided you have newer type phones they will still ring. Unfortunately that ring wire also acts as an antenae and decreases SNR. In my case, after I removed the ring wire my SNR went from 2db to 7db with a 10MB connection at night - as SNr on my line decreases into the evening/night.
So it's worth doing if you haven't tried it. The two wires that need to be there are 2 and 5, number 4(the ring wire) can be removed.