On Sat, 8 Feb 2014 09:24:37 -0800 (PST), David Paste wrote:

As I understand it, ADSL signals are passed along the twisted pair (is
that right?) telephone cables which are the same used for voice calls.
At a much higher frequency, of course. I am also under the impression
that the length of this cable is critical for service quality and
overall data rate (apart from junction / joint quality).

Pretty much correct. People underestimate the bandwidth that can be
carried over a twisted pair. 40 odd years ago broadcast video (up to
5+ MHz) was sent over half a dozen bonded twisted pairs, not far only
a mile or so but shows what could be done with the twisted pair and
the technology back then. This is ten years before the home computer
and 20 odd before cheap general access to the internet.

Would using coax for the last stretch to the subscriber's premises
provide any better quality of service?

It might but not without changing the kit each end. You couldn't just
join the two together, twisted pair is balanced, coax unbalanced.
Making a passive balun (balanced to unbalanced) convertor of
suffcient bandwidth (30 odd MHz) wouldn't be easy.

--
Cheers
Dave.