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If it's a decent size and shape, treating a room to be reasonably flat can be done for hundreds rather than thousands. Should be the first thing a serious listener does (or anyone working with audio) but it's not sexy and requires actual science rather than just splurging on the latest audiophile snake oil bullshit. It's also going to have much, much more effect than fancy cables or esoteric hardware - audible and measurable, which is a nice change in the audiophile world!
diminishing returns...