I just noticed from this article at http://forum.kitz.co.uk/index.php/topic,17065.msg313... that the stupid DLink DSL-320B-Z1 modem really does truly have bitswap off, it's not just a UI bug. The poster tested it and says it is indeed slow+crap (as you would expect) with it 'off'. So AA shipping them with defaults is a bad thing.
Fortunately I have doctored all of my modems anyway. You really do need to doctor yours - go into the web admin user interface on the device and change this setting. I have written this up in the AA support wiki article about this modem but I doubt the changes have yet been ‘approved’ that is made visible by default. There is presumably some way of seeing the latest version of the article if one digs around?