A number of the Head rackets have YouTek technology and they are characterised by the YouTek logo just left of the T-joint. A number of the same rackets on the other hand don't have this logo even though they have the same name and description. Is there any difference between the rackets in terms of their behaviour? The material composition seems to be exactly the same. Could it be that one type refers to older stock and the YouTek stock came later? Thanks.