Not sure what else you could try.
Have you tried connecting your Mac to the AEBS using ethernet? That'll give you a baseline to compare against connecting directly to the DSL modem. I would expect a small drop in bandwidth when connecting to the AEBS ethernet.
If using ethernet via the AEBS results in a big drop, then the problem may be that your AEBS is dying.
story - we used to use a Netgear router. Slowly over time, our internet speeds were getting worse and worse. Eventually, it was unbearable (<1 Mbps). I decided to check it all out. When I connected directly to the cable modem, I'd get 15 Mbps. When I went wifi, I'd get less than 1 Mbps. When I connected to the router using ethernet, I'd get 2 Mbps. Turns out that the Netgear was dying.
Another thing you might want to try is to turn off any encryption. There's some overhead involved with WEP/WPA/WPA2. See if that helps.