How to invisibly join superwash singles yarn? I know of several methods of joining a new ball of yarn, but I have trouble with some long-repeat colored yarns available as singles, be it thin ones like Amitola or even thick ones like Schoppel Reggae. 


Knots show up, especially in lace or reversible knits. 
Russian, braided, etc. joins require at least two strands of yarn. 
"Spit and rub" is the traditional method for lace, but only works with feltable yarns. 
taking the two ends together has never worked well for me. The ends slide against each other, creating too small an overlap, and the overlap knits up really thick and noticeable. 
leaving a tail and weaving in the ends works somewhat, but the stitches around the change become very loose, and the weaved ends are visible in some kinds of knitting (lace, rib). 


Is there something I am missing? How can I join the yarn invisibly?
Superwash single is a somewhat tough one, yes.

In those case, I usually start ten or so stitches before I change the yarn, I use the same technique as when doing jacquard to have the new yarn follow in the back of the work, I knit one stitch with both yarns, and knitting with the new yarn, I have the old yarn follow the stitches on the back for like ten stitches.
On the next row, I try to have the loose yarn follow on the back, so that it holds more.