s/good/accurate. One is a subjective quality, the other is not. It's the same reason many like Koss headphones. They think they sound good. On the other hand, they tend to 'colour' the sound, so they're not accurate.
Every headphone colors the music in some type. You'd have to search very far and wide to find a near flat headphone. Sennheiser has a veil at top with base emphasis, Beyerdynamic often lacks bass, Grado is very bright, many Sony's are bright, AKG is close to to neutral but lacks bass. Bose (yuck) has only one note of bass.
Koss is actually not a bad headphone manufacturer, their KSC 75 is often highly praised in audiophile circles, those same circles who argue over if EQ is right or wrong. Their A250 is a highly analytical can. Their Portapro is praised and many like the UR40.
Audio quality is subjective, but there are certain things I believe all headphones or sources should have. Detail, impact, sound stage, and clearity. The X-Fi lacks detail and sounds a bit muddy.
But, audio is audio, and opinions will always vary.