I seem to have done something odd at some point or else the web ui is lying to me.
The web ui just now in the broadband > advanced adsl-related settings page was showing all of the 'capabilities' in the first group set to OFF, so no bitswap [!], no trellis [!!], no phyrexmt u/s nor d/s, no SRA. I didn't make those crazy changes in the web ui! I fixed them and backed up the config.
(I diffed the saved config XML against a previous version and I could see that a couple of likely-looking elements had changed, they are ascii decimal in the xml, look to me like possible bit sets as after the change one was 0xC0000000 or something like that. If I am seeing what I think I am seeing the bits are perhaps inverted-sense, as in the web ui I had a fair few options set to ON, so don't expect to see a mere two bits set. Anyway, it looks as if I successfully changed something relevant and I managed to save the change, since the name of the changed element looks very plausibly relevant.)
I can't seem to get SSH to work for me. One app, my usual SSH client, just tries to connect and then terminates, another app that speaks ssh hangs when I try. SSH is enabled according to the web ui. Telnet works. I'm suddenly realising, it's probably the NAT that my Firebrick is doing to get to the B10A. Is that possible?