70db attenuation is very high and the route of the problem. check your internal wiring before the master socket for a small connection block. It's the size of a small box of matches.
If the 70db load is not due to a very long lenght of line then it may be due to an rf2 filter fitted before the master socket to reduce backround radio interference affecting your normal voice telephony. Have a look at the following:
http://www.kitz.co.uk/adsl/btsockets.htmScroll down to Block terminal connectors and look at the bottom at the rf2. If you have such a block then speak to your isp about getting it changed for an rf3. The rf2's where made before broadband came about and they filter the broadband signal heavly. With a typical phone line the limit in achiving any broadband connection would only lie around 2.1 to 2.2 km.
Might not be the problem, but certainly something to look for. with such a high load I hope it is the problem