Untreated rooms don't sound great. Untreated rooms using drywall sound pretty awful.
That's an artifact of over-tech. The decorator trend towards the sterile, clinical aesthetic, over cozy, functional/livable. The same would hold true of an empty room with wood walls too, and even multi layer plaster, to a slightly lesser (or different, at least) extent.
The current trend of decor with say, two channel enthusiasts, shows an empty room amps/separates arranged on the floor just so , speakers, and one or two seats in what amounts to an oversized room for even 6 listeners, and a centrally located, token rug.
Once you add in sufficient clutter to make the room actually inviting, acoustics change. My listening room is @ 12' x 20' with drywall over concrete. Has a sofa and a love seat, desk, tables, chairs, books, vinyl records, multiple pairs of speakers in rotation, and a rather fluid assortment of life items on any given day. Add 2-3 people, and it ends up just about perfect.
Most noticeable improvement from one moment to the next? Adding wall-to-wall padded carpet, and again when moving the furniture back into the room. If you have music playing while the work is being done, you can actually hear an incremental, not-so-subtle difference, as it occurs. I will never have a critical listening room without substantial and abundant padding, ever again.
