If you've read my previous post, I mentioned that the best way to equalize is to
subtract, not
add. What you're doing here is clearly adding, and as such it opens you up to some distortion issues. But like I said,
why use Windows Media Player? WMP is crap by all means.
This is what you should do. Instead of raising a frequency above zero, lower all other frequencies below zero instead. That way, you leave some headroom, which prevents distortion. The EQ there is the iTunes default EQ; it's not that good, but it isn't crap either. Probably the best EQ would be
Viper4Windows (or Electri-Q if you use Foobar2K).