Didn't they say a while ago that the
Plusnet line speed was now deprecated and may not be updated?
Re the problem itself, I think you may be suffering from the same issue as myself on AA. I have encountered it in two different circumstances. I just haven't the energy at the moment to tackle AA support, as for both occurrences I've received a script-monkey response on my first report of it.
Not what one expects from them.
The first occasion was when my line was banded. The connection speed that BT Wholesale reported to AA Radius server was considerably below the sync my modem showed. This was the case with
five different modems. The IP Profile was based on this low figure.
Once the banding was removed the problem went away. IP Profile reverted to 0.9669 of sync, as expected with G.INP active.
I had a new router in play, and after a while I went through the reduction to a 5dB, then 4dB then 3db target SNR Margin, with corresponding higher sync's.
As soon as the target SNRM dropped from 6dB the problem (or should I sat "the effect") recurred. I went from the IP Profile expected from the sync to one more like 0.94 of sync.
Result - throughput is not much different on 71.3Mbps connection from what it was on 63Mbps or whatever it was at 6dB.
In my case at least, something is wrong between Openreach and the line speed that BT Wholesale are seeing when the line is
not on "normal/default" Openreach settings.
I got fed up with AA when the banding was in place, but it wasn't really worth a fight until that was removed. Since my SNRM started being reduced I have contacted them once and got a "Can we have a screen-shot of the router GUI stats?"
when my ticket contained a full xdslctl info --stats listing 
, and "Have you got another router you can try?" despite in the same ticket saying this had happened before using 5 different modems/routers when the banding was in place.
I am close to being wide awake enough and with nothing more important to deal with, so may be getting onto them again soon. But if I'm right about this mis-reporting of the sync soemewhere in the Openreach >> BT Wholesale >> IP Profile clac and ISP Radius report it isn't going to be fun

.
Good luck with Plusnet if you have the same problem. Ask them what BTW are passing them re the connection speed! I bet it's around 60720Kbps.
My broadband basic info/help site -
www.robertos.me.uk. Domains, site and mail hosting -
Tsohost.
Connection - AAISP Home::1 80/20. Sync 71288/12440Kbps @ 600m. BQMs -
IPv4 &
IPv6
Edited by RobertoS (Sun 06-Aug-17 22:33:38)