You need a headphone amplifier. The Sennheiser HD600 series is not and was never intended to be a headphone for normal people to hook up to their computer. They were purposefully built do be very high end headphones for audiophiles to hook up to dedicated headphone amplifiers.

Now that expensive headphones are trendy and the price has come down a little, normal people want to buy them. But unless you do all of the other stuff you need to do to use them, they're going to sound pretty "meh" and you'll be sad that you wasted all that money.

This is why I use Grados. Even though they're high quality headphones, they work just fine with pretty much anything. Better with a good amp, sure, but great with a phone, computer, whatever.