(n.) Quality or state of being mellow.

affability agreeability agreeableness amenity amiability amicability bassness bliss blissfulness compatibility complaisance congeniality cordiality deepness delicacy downiness easiness enjoyableness featheriness felicitousness flossiness fluffiness fullness geniality gentleness goodliness goodness graciousness harmoniousness hollowness insolidity leniency lowness mellifluousness morbidezza niceness nonresistiveness nonrigidity plangency pleasance pleasantness pleasantry pleasingness pleasurability pleasurableness pleasure pleasurefulness plushiness pulpiness rapport resonance richness satininess silkiness softness sonority sonorousness sponginess sweetness tenderness unsolidity velvetiness vibrancy welcomeness