If you have music recorded in stereo (2.0) reproducing it with surround (5.1) is either combining the two for center, and doubling it in the rear with all lows passed to the subwoofer, or it's not actually using them all. Regardless, you're altering the input quite a bit from the original 2 point source to a new multi-output setup.
2.0 or 2.1/2.2 with a good musical subwoofer is going to be a better option for reproducing things recorded in stereo regardless, especially when you go from the little drivers in a surround bundle, to properly tuned (for music) speakers with bigger better drivers.
Without getting into too much detail, the crossovers for music and the crossovers for theater are generally designed quite differently, so they perform well at their intended job, and adequately at the other. Some crossovers allow for a very versatile speaker, but these are generally more easily found in the higher end, neutral sounding gear.