Regarding Nopoops comments about the pitch, he's spot-on. But not because of the headphones.
If you play the engine sound from windows, it sounds fine. However it sounds horrible in AH2 because you only play full pitch when at WEP! That's right, when you're not at WEP you get a dull, low pitch sounding engine. Have to take that into consideration when mixing it. Have to mix it too high, then have it get lowered in-game to normal throttle.