Haven't read all this thread regarding the REIN issue, but to clarify a couple of your points.
A 6dB profile on your DS is pretty much a default setting and IMO, perfect for stability. If you were to try a 9dB profile, expectations are you would lose approx. 0.5-0.75Meg.
As your ISP guys have commented, by tweaking parameters you are 'masking' the issue at hand. It's all down to what the EU is prepared to put up with ??
I can say on most REIN cases I attend, the SNR will have been tweaked high, and the INP (Impulse Noise Protection) will also have increased from default. There is always the odd one though that goes against the grain.
Best of luck.
