Originally Posted by Lambsscroll
What if the coach says its because of the strings?
Well, hard to argue with that, if that's true. If that's the case, then no more needs to be said on the matter of technique.
If the coach has judged the technique to be sound, and the performance between two distinctly different strings provides such clearly different results, then we should definitely look at the strings with increased scrutiny.
All things being equal, if it is the strings, then I would say the following:
Head Sonic Pro is a considerably softer poly than Genesis Black magic. Not only is it softer, it doesn't hold tension as well as the Genesis does. This means that a few hours after stringing, plus hitting a couple hundred balls, a 40lb string job will degrade into a 17 lb string job. What kind of control does one expect with strings @ 17 lbs?
Genesis black magic is much stiffer than Sonic, so its got a lower deflection off the racquet face. This means better accuracy, but less spin. It holds tension better, so a 35lb string job will degrade into a 16 lb string job.
TL/DR: The problem is not with the string, but with the ELT method of stringing (extra low tension). Personally, I don't see the point. I recommend jumping up to around 54 lbs. Once they are strung at "proper" tensions, then you can really begin to analyze the performance of each.
But honestly, you can't really play competitive tennis when your strings are @ 16 lbs of tension. That's just way to low.