1973 was really (in retrospect) the peak year for quad - at least commercially. The quality of mixes got better from 1974 onward, but I doubt there was a year when more quad product was released than 1973.
A few highlights:
Warner/Elektra/Atlantic released their first CD-4's, a batch of 25 titles, in September
Stax released their first quad product on Q8 and LP (four Isaac Hayes albums and a Staples Singers album)
United Artists/Blue Note released nearly a dozen Q8's
RCA went full bore with their CD-4 program, even trying to go single-inventory for the first half of the year
..and other major labels really hit peak quad production, including Columbia/Epic, ABC/Impulse, and A&M to name a few.
Musically, I think it's also worth noting that 1973 is really the year that all of Miles disciples kind of "graduate" and jazz fusion comes of age:
Herbie Hancock's
Headhunters
Mahavishnu Orchestra
Birds of Fire (and
Between Nothingness & Eternity Live)
Return to Forever's
Hymn of the Seventh Galaxy
Billy Cobham's
Spectrum
Weather Report's
Sweetnighter