Most sound cards allow setting the output levels and speaker configurations
via software that is accessed from the Windows Control Panel. Please refer
to the sound card documentation for instructions on using this software for
setting up the outputs for 5.1 audio to properly drive the headphone speakers.
Also, note that some sound cards may have the bass and center channels
reversed, which will cause an apparent decrease in bass response. This
can often be corrected with a setting in the sound card control software.
You may need to experiment with different settings in both your sound card
mixer and the physical volume dials to find your preferred listening levels.
If you find that your sound card is unable to provide a strong enough
output signal, you may want to consider purchasing the Turtle Beach optional
HPA amplification adapter, available in the product's accessories section.
It sells for $49.95 plus S&H as a stand-alone item. (Optional Accessory)