You might not believe it, but an external DAC/amp makes a huge difference. No matter how nice of headphones you might get, they can only play the analog signal they receive. The headphone jack on a PC probably isn't putting out the cleanest, sharpest, fullest signal around - but a good dac/amp is putting out a good signal to your phones. Even on a high end machine, the speaker/headphone outputs and the digital/analog converters that precede them are going to be among the spots manufacturers like to skimp on. It won't sound "bad," not unless something is defective, but there is a huge difference between "OK" and "amazing," and a new dac/amp can definitely do amazing.