1. Yes, absolutely crucial but for two reasons, not just one: Firstly, yes, speaker performance is defined by the room acoustics. Typically, a $1,000 set of speakers in a well treated room will out perform a $20,000+ set of speakers in an untreated room. Secondly, commercial recording/mastering studios are NOT designed to be dead (anechoic), they do typically use some absorption to reduce room reflections but they usually employ at least a fair amount of diffusion. Diffusion randomises room reflections rather than removing them, which stops the reflections from interacting and changing the perceived frequency response. This too is a crucial factor because music is mixed and mastered in these rooms (with their neutral room reverb/acoustics). In other words, the music has been designed to be listened to in a room (with room acoustics). Listening to music on headphones obviously largely eliminates room acoustics, the music therefore sounds much drier (less reverb) than intended. The result of effectively less reverb is that the music sounds more "in your head", more separated and more detailed than was intended. Some people like that, others prefer accuracy (to hear what the artists intended).
2. I would say that's certain rather than "possible". It's not a question of "if" the setup/room acoustics are "sabotaging" speaker performance, it's a question of "how much". Typical rooms in houses are roughly cuboid shaped which is unfortunately the absolute worst possible shape acoustically.
3. That's simply impossible. I'm not knocking your 305's, I've used them myself and IMHO they're about the best you can buy in their price range. There's a bit of a misconception that Nearfield monitors solve room acoustics issues, this is not true. They can significantly reduce some issues but they can also cause others; typically they're placed on a desk, which acts as a very close, big reflective surface and that's very bad acoustically.
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There are a few photos/videos in this thread of systems which some members seem to be impressed by. I'm not impressed, quite the opposite in fact, to me they appear ridiculous. They look like the audio equivalent of someone who's just stuck a new V12 Ferrari engine into a Ford Fiesta. It might give you great "bragging rights", sound awesome and out perform a unmodified Fiesta but it won't perform anywhere near an actual supercar, it won't even perform as well as a relatively cheap stock sports saloon! Those audio systems are designed by people (and for people) whose passion is audio equipment, not those whose passion is audio performance. IE. It's for audio-equipment-philes rather than audiophiles! Here's a photo which illustrates my point above (about diffusion) and is a real audiophile "supercar". Notice that it's not about massive speakers, it's about very high quality, appropriately sized speakers and the effort put into the acoustic treatment:
G