> what do people find works well on long ADSL lines?
I have ultra-long lines - downstream attn. 66 dB. My DLink DSL-320B-Z1 modems work superbly well on ADSL / ADSL2 / 2+. That doesn't fit in with your single-box thing though. But I'd recommend these separate modems, as they're superbly aggressive performers and dead cheap (< ~£20 ) to replace, and if you get a lightning strike then there's some small hope that just the modem will be taken out, not the router as well. Mine came from the AAISP shop, correctly preconfigured to be in modem-only mode. (i.e. not trying to be a router, in which case they would be utterly crap, full of security holes and bugs according to A & A.) Definitely worth getting them preconfigured, saves you a small chore.
Following a good tip from Burakkucat, mine are configured to be in ADSL2 mode, not "auto" allowing ADSL2+, which we hope narrows the window dropping the unused higher frequencies in the input, so reducing input noise. That's the theory anyway.