I managed to leave out a few prime examples from the San Francisco Bay area:
- Stevie Nicks & Lindsey Buckingham. Prior to moving to L. A. and securing a record deal, this duo were members of The Fritz Reiner Memorial Band (shortened to simply Fritz after being being challenged by the conductor’s estate). I saw them live only once---opening the 1968 Santa Clara County Folk/Rock Festival. If you look hard enough, you can find a pic of Fritz on stage at Mother Butler High School, Stevie in a prom dress/formal wear (the Catholic girls school had a serious dress code). The duo’s sole 1973 album on Polydor now fetches somewhat serious money, in spite of it being not very good (imo, of course).
- Prior to joining Copperhead (John Cipollina’s post-Quicksilver Messenger Service band), bassist/songwriter/singer Jim McPherson was in San Jose band Stained Glass (two albums on Capitol Records).
- Prior to joining Moby Grape, Skip Spence was a member of The Jefferson Airplane (first album only). Moby Grape was a serious step up from The Airplane.
- Prior to launching a solo career, Dan Hicks was the drummer of The Charlatans (one album).
- Though I much prefer The Sons Of Champlin to Chicago, joining the latter no doubt resulted in a serious career boost for Bill.
And then there is Tom Petty, originally a member of Mudcrutch.