Hd681 series professional monitoring headphones the hd681 series headphones offer 3 distinctive sound profiles each with its own character the hd681 features an exceptionally deep low frequency the hd681f has a flat extended frequency response and the hd681b has an excellent balanced character and powerful bass punch as for the sound image presented by these headphones the hd681 gives you the feeling of actually being on stage with the artists while the hd681f makes you feel as if you s