does a subwoofer kill stereo sound?

and, with some effort, could also improve same.

When I shut my sub off, the stage gets narrower.

Sub is playing well with panels.

One of the best things you can do is add a powered sub or subs to a system. If done correctly, it frees up the amp to power the mids and highs adding much better performance.

I have full range Von Schweikert VR4sr's driven by Audio Space Ref.3.1. I felt the bottom end was mostly missing with only 42 watts per channel. I called Albert and he suggested his new subwoofer with a no questions asked 90 day in home trial. The differece is substantial. My soundstage actually opened up a great deal with much more inner detail audible. The synergy between the speakers and subwoofer makes the sub "disappear". On recordings with lots of bass, the sound is full and deep. On recordings with less bass, you can't tell the sub is in the room. As with all things audio, your mileage may vary. Happy listening.

I'm with MagFan an Stringstee, on this, I think a sub always adds to a system, no matter what the speakers.

I am shocked when I turn off the sub(s) with my Maggies, how much bass they put out. Then when I put the sub back into the system (I cross it over at 41 hz) I'm shocked again at how deep the image gets, and how much slam there is.

It does take time to set up a sub properly though, so they are not muddy, but it's worth the time.