1) Agreeing 2) Harmonically aligned 3) Harmonious 4) Harmonized 5) Jibing 6) Like a concert piano 7) Like harmonious bands 8) Like new pianos 9) Like well-maintained pianos 10) Not clashing 11) Not flat at all 12) Not off-key 13) Not sharp at all 14) Not sharp or flat 15) Not sour 16) On key 17) On pitch 
• (v. t.) To intone. Cf. Entune.
In·tune' transitive verb To intone. Confer Entune .