Best Acoustically Untreated Room You Have Heard?

I have been researching room acoustics again in an effort to improve the sound of my system. Most webites give general information on the basics (resonance and reflection control) and some examples of properly treated rooms. The problem is that most of these rooms are just plain ugly (room with eleven tube traps) and involve too many compromises.

I'm curious to know if anybody has heard a stereo system in a room that did not have any (or very little) acoustic treatment that sounded fantastic. And more importantly, what factors contributed to that sound? I suppose that the furnishings acted as diffusers and absorbers, but what were they? Or maybe it was the shape/composition of the room and or ceiling?

Thank you in advance for any input.