Re the atten.... Not sure sorry, perhaps someone like walter may be best as he deals with quite a lot of long lines.
Around here we dont have many real long lines so its not something I personally often see. At a guess - and it is a guess - it would be those routers which display more than 63.5dB on the downstream (this may be easier to search for)
Upstream SNRm variations though should be easier, afaik most of the BCM routers (aside from say BT HH or other ISP b@st@rdised firmwares) should do so.