does a subwoofer kill stereo sound?

Some will tell you it will degrade the purity of the stereo image. Others will tell you how it can open up the sound. I'm with the later camp, if you can get a good blend. I have used two subs, now I use one near a corner all the way to one side. You would never know where the sub is, it integrates completely with the soundfield. Some rooms are not fullrange speaker friendly, in some stand mounts w/sub ultimately work the best.

Most "fullrange" speakers with really good bass response are very expensive, even in high end audio $$$. Ohjoy40 Kevin may have the luxury of a listening room where a speaker's mid/high sound and bass response both work in the same placement. I would guess many of us are not so fortunate as to have the optimum listening room and to afford a great fullrange speaker.
The subwoofer "compromise" is often used even with speakers that have impressive low end. A properly set up sub should not be heard- only missed when turn off.