I always thought line attentuation stayed pretty much static and was measured at a set frequency like
the 300Khz you mention, only the noise margin changed...
I was surprised the value changed so much between routers !
I've since learned the attenuation can change when changing modes ADSL > ADSL2+ for example
for the reason you mention.
Looks like my D-Link router is going to be delayed as it's coming from a company in Scotland with a
delivery backlog due to the recent weather.
If I hadn't bricked my Netgear DG834v4 (Broadcom) during a firmware upgrade last year I'd already
have plenty of line data to work on...Grr.