In the same vain as a Mingus ballad I would reccomend Wayne Shorter's tune 'Fall' which you can find on Miles Davis' album 'Nefertiti.' There is a horace silver peice called 'Lonely woman' which you might like. Blue in Green, a Bill Evans tune, although generally credited to Miles, is a great one. Just a few ballads that have the sound I think you're after.
