Feedback from the keller-sports.com team:
What I really like about this string: It is very fast, offers a very good control plus the strings hardly slip. What I don't like is the hard feedback of this string. SOme people might like this touch and call the feedback "direct" or "firm". Me, I am a player who doesn't like such a hard feedback but indeed this is a matter of taste. Whereas I gained the impression that this hard feedback somewhat depends on the colour, thus I had the feeling that the Sonic Pro in orange and especially in black were a bit softer than the white version.
Result: The Sonic Pro is avery fast string with good control and a very direct feedback, that is especially suitable for hard-hitting tour players.
Co-polyester string from Head for hard-hitters who attach importance to tension stability, good acceleration and a long life time. Thanks to the brand new polyester technology, this string offers maximum power, very good control and a markedly longer life time than normal polyester strings.