I've owned two DACs previously. A Xonar DX and then a D2X.
Going from a built in chip to a DX was remarkable, added quite a lot of clarity between frequencies. DX to D2X introduced tighter, stronger bass, and slightly better resolution. The D2X to the DAC-iT was the best upgrade I've ever made in my 6 years of audiophilism (beats out going from RS1i's to LCD-2s). My music just sounds so pure now, like there's nothing wrong with any part of it and there are no conflicts for sounds to be heard, it's just perfect, clean sound.