Speke
DACS does not affect modem speed. It simply uses the spare bandwidth (of which there is a huge amount available) on the copper wires. It's an urban myth I'm afraid.
Line distance from the exchange, the quality of the line and the telephone exchange equipment have a major bearing on modem speed. Also, distance from your ISP counts sometimes. The old chestnut of "increasing the gain" (ideally to "3" on BT lines) on the line (by your telco) can also work but....as it is the equivilant of turning up the volume, it can also excacerbate any line noise problems etc.
I find that disconnecting my telephone extensions increases my max connection speed from 26400 to 50666! Your home/office 'phone wiring can, therefore, also contribute to poor connection speeds.