The use of the thumb on the right (or which ever is your picking) hand adds soo much warmth to tone. I noticed the thumb was only mentioned in the bass comments in regards to slapping. Using only the thumb for picking on bass sounds great and it also forces you into a different feel. I believe Family Man used his thumb. Also, I know Charlie Hunter uses his thumb (has to) and I believe Pino Palladino also did for Voodoo. Hence some of those really warm, muted bass tones.

For guitar the thumb is killin as well. I find that guitar has a really weak, tinny sound to it. But the use of the thumb for picking really helps. Probably the best tone to ever come out of a guitar was that of Wes Montomery. He used his thumb exclusively. Thats why those octaves of his sound so damn good.

If you are using a pick, try flipping it over to use the rounded side instead of the pointed side. This also adds some roundness to the tone of guitar.