Well you should be getting around 3.2 to 4.6Meg for connection speed at that atttenuation.
Is this using the test socket, i.e. remove the removable part of the master socket (disabling all your phones)?
The suggestion is either
1. The modem is lying about the 50dB attenuation
2. You have noise on the line, probably from sky/phone/alarm/ringer connected without a filter
3. You have lots of extensions with no phone connected, but the ring wire is connected picking up excess noise