Guys,
It would appear that i have an issue using any new device based on Broadcomm. I first noticed and reported it when using the Netgear D7000. However, i have exactly the same problem with the new Billion 8900AX-2400 and now the BT Smart Hub.
In summary the issue is with decreasing S/N over time due to bins being permanently marked as unusable.
See my post in this thread.
http://forum.kitz.co.uk/index.php/topic,15601.msg294650.html#msg294650Netgear concluded that it was a broadcom issue and there was nothing they could do. Having had this on the billion and now the BT Smart hub, i believe them.
It is usual for my S/N to vary by around 1 dB at night due to external "interference". This can be seen in the tone graphs -ie S/N is reduced at certain bins and hence swapped out. With the HG612, billion 8800AXL the same thing happened, but the bins were re-allocated when the S/N recovered.
I have an open fault with BT as a line test showed an issue that has now been fixed. I have kept the issue open and have the option of having an engineer visit. However, im not sure if this would be worthwhile.
I've attached a graph of S/N performance between the Billion 8800 and Netgear D7000 as an example. The start and end of the graph is with the billion, the middle part is with the D7000. The peaks in the middle are due to a reboot to recover the S/N.
With the BT Smart Hub, i started with a S/N of 7.8 dB, three days later i'm at 5.4dB