i think The Doors were on Elektra?One Elektra (our first!)
Two Atlantics
One Warner Bros.
my number one Elektra Quad for the Rhino Quadio treatment would be Stardrive Featuring Robert Mason "Intergalactic Trot".
closely followed by a David Gates' 2-fer (his later Quad album is so short and sweet it's almost an E.P...! ) and then Sergio Mendes' Self-titled 1975 Quad and a Carly Simon Quadio Box Set.
for Atlantic, either of the remaining 2 x Spinners Quads, Duke Ellington, MJQ, Aretha and Herbie Mann's "Reggae" is a lovely one as well.
there's a fair few WB label Quads deserving of Quadio kid gloves.. my main one as it's such a fun active mix and it's so scarce being a super late entry U.K. CD-4-only release is 1976's Deaf School's "2nd Honeymoon".. any James Taylor Quad, of course, maybe a 2-fer or box set if more unreleased Quads are unearthed?
Arlo Guthrie.. Maria Muldaur.. Dionne Warwick(e)... a Tony Orlando and Dawn Elektra 2-fer could be cool... Hot Tuna.. more Alice Cooper and Jefferson Starship.. a Souther-Hillman-Furay Band 2-fer (nice mixes and sound quality iirc).. still loads to do!