I dug out the two books that I'd seen connecting the Dead with Stockhausen: Alex Ross' excellent The Rest is Noise and Mark Pendergast's The Ambient Century both make passing mention of Stockhausen's lectures at UCLA in 1966-67 that were attended by "members of" the Grateful Dead and the Jefferson Airplane (Ross 474, Pendergast 54). While I'd bet that Phil Lesh was the Dead's chief representative (he had already been studying this stuff prior to joining the Dead anyway), that still seems like a thread worthy of investigation. Stockhausen himself also apparently took in some concerts at the Fillmore.
LIA also pointed out that Garcia himself shows no sign of having listened to anything like this, whereas Lesh and Weir have mentioned classical influences in interviews. At the risk of sounding like even more of a dilettante, I'll suggest that music like Stockhausen's Kontakte may not need a lot of repeat listenings to completely re-wire one's sensibilities, particularly if one is possibly under the influence of a psychotropic substance like LSD. Unlike, say, John Coltrane, who's impact in live performance was certainly just as visceral and potentially life-changing (cf David Crosby's great story), figuring out exactly what his band was doing probably required you to sit down and listen repeatedly to the record -- which is apparently what various members of the Dead did, Garcia included. Certainly there is far more to music like Stockhausen's than one will ever appreciate at first listen (not that I know what it is yet), but I don't know that much of the internal workings of something like Kontakte would have really impacted the Dead. My guess is that, outside of Phil Lesh (who, again, was already well-studied in this kind of music), no one was sitting down with the scores for this stuff and incorporating those techniques into the Dead's own musical approach. Outside of Anthem of the Sun and Aoxomoxoa, there really wasn't much experimentation with tape loops and electronics, was there? And by 1968, that influence was probably more directly traceable to the Beatles than to Stockhausen himself. I'm also putting aside Seastones and Phil & Ned for now; clearly Ned Lagin owed a great deal to Stockhausen.
I would think -- and I'm just guessing here -- that someone like Garcia could have spent a couple of evenings bugging out over a record like Kontakte and then never intentionally listened to it again. But that could have been enough: it still could have had a profound impact on his still relatively fresh and still-expanding conception of music. I can see how just a couple of listens could prompt the kind of thinking that led to the Feedback meltdowns of the 60's, the sparse interior jams in the 1969-1970 era Dark Stars, that sort of thing.
edit: I went back through David Malvinni's The Grateful Dead and the Art of Rock Improvisation (2013), and he touches on Stockhausen a bit in one of his chapters on Dark Star, although not really in a way that answers any of my questions. In short:
- Tom Constanten actually studied with Stockhausen in 1967, prior to joining the Dead. That experience rubbed off directly on Anthem of the Sun [although maybe more in theory than in practice; I always had the sense the Anthem sessions were pretty chaotic].
- Lesh was a devout admirer of Stockhausen, modeling his own early (pre-Dead) orchestral writing on Stockhausen's work. Lesh also "ran the controls" for performances of Stockhausen's tape compositions.
- "'Kontakte' is spatial music with swirling electronic effects, based serially on the placement of speakers in a room for its full effect; it clearly is the sonic godfather of the Dead's concept of 'Space,' a rhymthically free region relying on electronic effects both within sections of pieces like 'Dark Star' and as the middle point of the second set after Drums." (Malvinni 107)
- There's another mention of members of the Dead attending Stockhausen's 1966-67 lectures (at UC Davis, according to this source, not UCLA), but no further details.
- Stockhausen redefined his music as "intuitive" and "beyond improvisation," embracing the belief that humans were on an evolutionary cusp of experiencing universal consciousness, telepathy, etc. The Dead (and Lesh in particular) embraced similar ideas of group-consciousness (both with each other and with the audience), though that seems more like a parallel aesthetic and less like an influence.
- Stockhausen and his students may have attended the 4/24/72 Dusseldorf show… that's according to Rock Scully's book, though, so a grain of salt for that one.