I agree- it's an extraordinarily large (if not on-topic) topic/can-of-worms/what have you. BIS apparently was able to find lots of material for their Sibelius (once one of the most sung of composers) edition much of which was almost unknown without scraping the bottom of the barrel, I gather. There's similarly, I understand (in both cases I have heard
some of this music

) huge quantities of quite good but underplayed Haydn, also for example...
(And don't get me started on the Bach cantatas, once quite a lot more underrated than now, still much underrated anyway -- but at least in relation to the Bach cantatas there is an
outstanding website with full and vocal scores, recordings or sound samples, and much else.)