Apogee audio interface better than a DAC?

I recently installed an Apogee One on my iMac so I could record guitar in Garage Band. Which it works well for. What I was surprised to find, however, is that now my studio monitors sound much better. (They were previously just plugged into the computer's stereo output jack.) In fact, out of curiosity I added Decibel to replace iTunes and man! I'm not certain, but the iMac/Apogee/Jamo system in some respects is superior to my Audio Research / NAD / Green Mountain Audio main system.
The computer system is very clear and revealing, which I guess a monitoring system is supposed to be. But it isn't off-putting or headache-inducing like I would expect. (Although I haven't sat there and listened for even a full hour, so maybe that's to come.)
Has anyone else noticed their relatively inexpensive home recording studio is a top-flight hi fi? I'm wondering how to use the Apogee in my stereo, but I don't think it's possible.