Hrm I just stumbled on this thread about SNR Margin today.
My 504T reports mine as:

SNR Down: 10
SNR Up: 9
Line Atten Down: 35
Line Atten Up: 17

From what I can tell from the first post, my atten is fine, but my SNR is borderline terrible.
However, my connection seems to be OK, just that downloads from some sites are slow. And BT is only doin 30+kBps instead of my usual 50+kBps.

What could have happened? Recently my Netgear got fried (presumably by lightning), so I'm wondering if maybe it's possible the wire from my wall to my modem is defective adi?