After rewiring as suggested in the last post [*], our speeds jumped from about 75 Kbps down, to about 200 Kbps down. Which is a huge improvement, even though the advertised speed is 768 Kbps. I anticipate that getting Verizon to care about a download speed 1/4 of the advertised, rather than 1/10, will be a lot harder, but I will try. Some other time.
[*] Splitting the upstairs phone jack, B, close to the wall; B1 goes straight to the DSL modem, B2 is filtered at the split and then goes into the answering machine/telephone. (We dropped one of the phones that had been upstairs). We also tested plugging the DSL modem straight into the jack without anything else split, and got the same speeds.
Thanks to everyone who responded!