Good question, ajc_trw..! :upthumb
fwiw, the 1st album by "Chase" being sold on iTunes is said to be SQ-encoded.. though I've read that the process of compressing the music to MP3/AAC format can be detrimental to the SQ-encoding, I've not tried the iTunes offering thru my SQ decoder myself so can't say either way. I would try it but the old "Chase" SQ LP decodes so well thru the SQ Vinyl Surround Master I just don't feel the need (that and I prefer Chase's last album "Pure Music" anyway.. "Pure Music" is one of the best Columbia/Epic Quads imho, its fab.. the iTunes of "Pure Music" is said to be SQ Quad encoded too.. but again I've not tried it, the old Quad record is just too good to care!).
I'm sure there are plenty of others on iTunes.. possibly look out for albums where there was only ever a matrix encoded Quad/Stereo compatible release (like "Dan Hicks & His Hot Licks' Striking It Rich"; afaik all versions of that should be QS-encoded).. probably quite a bit of Jazz like that too that was only Quad (Impulse? or any Rudy Van Gelder CTI productions.. there's a couple of Deodato albums iirc) there are members here very into all that, fizzywiggs41 is one, I'm sure they will find this thread and chime in before too long.
I went thru a phase (pun intended!
) of trying out iTunes downloads in ProLogic II and also looking out for phasey-ness in Stereo playback and that kinda wide stereo front soundstage of SQ (thanks for the tip on that, Dave!
).. I've forgotten more about what seemed like it might work out in SQ in the time since I actually just went the whole hog and got into buying the legacy Quad records because exploring that has been so much more fun than fiddling about with iTunes (as time's gone by I've come to dislike the iTunes interface & functionality more & more, so I hardly use it much now).. anyway, good luck and have fun!