The Cisco 678 in in DMT will default to the max speed (8032down/860up) at every powercycle and/or reboot. However, in training to the DSLAM, it negotioates a working line speed, either the max the line signal to noise ratio will allow, or the speed programmed into the ATU-C port. As you're getting 256down/256up, I suspect that the speed is throtled by programming for the port at the DSLAM. Who provides your DSL line? If they only provide so much bandwidth, anything over that available from your ISP is wasted.