Onboard sound hardware is pathetic

I'm using an Asus P5GDC-V motherboard with CMI9880 integrated sound. Music (specifically trance, house and progressive) sounds pretty bad through headphones connected directly to the back port but half-decent through external speakers. The problem with headphone sound is the noticeable lack of bass and way too much treble. I've adjusted my equalizer settings to compensate but it's still pretty bad. Low-frequency distortion is also quite apparent.

Is this a problem afflicting integrated audio hardware in general (i.e. do I need to shell out a few $$$ for a dedicated card?), or are my settings awry?