Now here's an interesting term. Congested would normally apply to a health issue one might have but in audio terms it describes the way something might sound when it gets bunched up. It's funny how we use some health terms like congested or constipated to describe the way something sounds. Stiff, unmoving, wooden, mechanical. But let's focus on congested.
I think the easiest way to explain what a congested vs. open sound might describe would be like the difference between listening to a small quartet in a tiny space vs. a wide open venue, or like the difference of being in a crowded room where all the conversations bunch together in a complex soup, vs. those same conversations out in an open area.
I have heard congested tendencies in electronics, in particular preamps and amplifiers, but mostly in speakers and cables. One might also describe a speaker presenting a "thick" sound rather than "congested", and most speakers displaying a thick tendency on a single voice or instrument also seem to share a congested nature when multiple instruments and voices are presented.
I rarely ever hear a thick or congested presentation from a thin membrane loudspeaker such as a ribbon, electrostat or air motion driver. I hear it more as a tendency of conventional drivers, but not every conventional driver sounds thick or congested.
I also hear congestion in system setups, primarily when you place a pair of speakers too close to the rear wall. If you think your system might suffer from a bit of congestion, a good rememdy may be to give it a little more breathing space by pulling it out into the room a bit.
Cheaper than replacing your speakers!