All this talk about too much or too little spring tension makes a pretty good argument for the hydraulic unit, the HB125. That unit uses a small hydraulic cylinder, open at the bottom, similar to a hydraulic lifter, to dampen compression and a return spring to do the actual pushing up against the chain. The spring isn't all that powerful, acting only to keep the shoe pressed against the chain. The cylinder allows movement, it just dampens it and then returns to the preload, also a dampened movement.
When I installed mine, I was concerned about a number of things--too tight being the number one. But I followed the instructions, and there it's been for about two years now. I never had any noise at all, although the manufacturer mentioned that there might be a slight whine for the first couple hundred miles or so. Shifting is definitely crisper and neutral is a snap.
Anyway, if you haven't bought the M6, you might consider the HB125.