David I read your post with interest, as I have a compulsion to try out any new line on the market.

I too have tried Sufix Siege line. I my case I never really got beyond tying on my leader material (20 lb Siege to my usual 30 leader material). I generally use a shock knot. Anyway I wrapped both lines in tea towels in the privacy of my kitchen and took up the strain to pull the knot up tight. To my amazement I saw the knot very slowly undoing as the tag end of the Siege disappeared and the knot flew apart. If I was sitting on a beach and rerigging I would have just pulled the knot up tight with my bare hands and not have realised I had a suspect knot - until I got snagged. I then tried a regular blood knot with the same result - under big tension the knot slowly pulled apart. This might have been happening to you (and others) the knot letting go, or the line cutting thru itself as it slipped perhaps?

I had previously not tried Black Magic Premier Supple line figuring that it was average line bought in bulk with a Black Magic logo slapped on it. However, about a year ago, in a moment of weakness I made an impulse buy in Phil's fishing emporium in Levin - BM Supple 12 kg and gave it a go. It knotted well, it cast well, was flexible, and was durable (didn't harden with use and stayed smooth) and had good strength. Personally I really like the visibility of coloured line which this doesn't have.