^ This.
I love the concept of the jam band, but I'm not much of a fan of the direction the genre has taken.
In particular, many jam bands have taken to the "singing ability optional" blueprint popularized by the Dead. In fact, I think that is the modern definition of Jam Band now: extended instrumental improvs and poopiee for vocals. I'm looking at you, Phish. But there are many others.
In fact, there are a number of bands that I would consider "jam" bands who could play for 20 minutes no problem, but aren't considered Jam Bands because a) they had a good singer or two b) they wrote actual _songs_ that had appeal outside the genre and 3) their music transcended the social scene they were associated with. Jam Bands are almost exclusively a modern white hippie thing. While many rock and jazz artists and fans were hippies back in the 60s and 70s, that just can't be said now.
In rock, Cream and Led Zeppelin were most definitely jam bands. Maybe a little too rockin' for the hippies, tho.
In jazz, well, Miles has already been mentioned. I'd add Weather Report and the like.
My favorite Jam Bands are the ones most closely associated with the "San Francisco Sound" that they helped popularize. So, yeah, that's old school, but from where I'm sitting that's where and how the modern genre was born. Jefferson Airplane's instrumentalists were every bit as good or better than the Dead's and Marty Balin and Grace Slick were terrific as well, making the music far more approachable even on extendo jams. I wish there were more weight on that style in the modern genre.
Want love for a Jam Band bassist?: Ok, Jack Cassidy. Best of the genre I think.