Are you referring to the target noise margin rather than max stable rate? On LLU Plusnet were able to change the former on the control panel and it remained fixed. On IP Stream Max the target noise margin is dynamically managed by the DLM (Dynamic Line Measurement) kit at the exchange, Plusnet do not have direct control over that.
IP Stream Max also introduces IP Profiles which depend on sync speed. A
BT Performance Test will show the IP Profile, and Plusnet mirror that value as the
Current line speed on the website. LLU does not use IP Profiles.