About the 3dB thing, with AA you can set this yourself in the 'clueless' control panel, provided you are 21CN users not available to 20CN users. But rather than just going ahead and doing it, it might be better to talk to tech support about the feasibility of it, as the line might become unstable unless it has been well-behaved. todos: check are you 20CN / 21CN
The DLink is just a modem not a router, so it's no use without a PPPoE-speaking Ethernet router to go with it. Now I'm assuming that the Billion might qualify - does it have a 'WAN' Ethernet port? - but I'm not familiar with the Billion range apart from their good reputation. Could anyone confirm or denying that this router speaks PPPoE to a WAN port? From AA shop ready configured.
DO NOT USE THESE IN ROUTER MODE, AS THEY ARE CRAP ROUTERS!! (unless you program them to modem mode first yourself, which is easy.)
We also, as an earlier poster pointed out, need to check whether you ADSL2+ or ADSL1 ? How do we check for this? This would affect 'my' speed figures, as I was quoting ADSL2 figures for myself, not ADSL1. (If I were to give ADSL1 numbers for my own line this would be 2000-2400 when things are well lower figures when conditions are bad, and throughput which is 2 Mbps when all is good, 1.75 Mbps typical, 1.25 - 1.5 Mbps when conditions are poor). So my earlier post was predicated on the assumption of ADSL2 mode.
So a list of a few things to check.