60 - 70 db typically for well recorded CDs, even at the lower end of this range detail, stereo image, and overall bass/mid/treble all sound good (e.g. Lucinda Williams, Car Wheels; almost anything by Mark Knopfler or Roger Waters etc). I find some (less good?) recordings need to be played at higher volume to work for me, e.g I find that early Led Zep or Queen albums have to be played >90db to work for me at all. Maybe just that I got used to hearing that stuff really loud back in the 70s!
Also have found that as I've acquired better gear (particularly amp and speakers) that more moderate volumes work better for me they did with lesser gear.