My impression is that over-deadened rooms are extremely detail revealing. It sounds dead, not my cup of tea, but you can actually hear more detail in the recording. "Air" and "detail" are not the same thing to me. I like some "air" in the room and to my ears it always comes at the expense of some detail, but it’s a worthy trade because it’s just more enjoyable. Now if it’s unevenly deadened, which is typically the case, you might end up with reduced apparent level in the upper treble with the lower midrange and upper bass still reverberating like crazy, so in that case you can end up with a muffled sound. But if you over-deaden a room broadband it sounds exquisitely detailed, perhaps painfully detailed. Diffusion can have differing effects, depending on how it’s done. Diffusers that have long wells or cavities will absolutely smear detail if they are too close to speakers.
The thing is, somebody might still find that to their liking, and the word "detail" can mean different things to different people. Generally it has been found that listening rooms are preferred with an RT 60 of around 0.3 seconds. The RT60 should be shorter in in the mid range and longer in the highs and lower bass. A lot of home listening environments end up just the opposite. The walls and various room openings often leak the lower bass pretty quickly, and the carpet and furniture sop up the upper treble very quickly, leaving a lingering midbass and lower midrange. Typically the absorption should be focused there, where it’s most needed and not where it’s not needed. A midbass to lower midrange absorber with a treble reflector/diffuser ends up making a lot of people happy. I’ve had people tell me,( and I’ve experienced it myself), that if you put a TubeTrap too close to a speaker with the treble reflector pointing toward you it can actually make the sound in the room too bright.