A more sensitive and dynamic speaker will sound brighter than one that is not and yes, it is due to volume and ease of reproducing microdynamics, the M3's can swing easily from soft to very loud, and the less compressed the recording is the more you will hear it and have to turn the volume down, over-compressed recording much less so because of cause everything if even and loud. The sound from the M3's is louder than you think and it's due to how clean and distorted they are with no bass overhang or boom on every recording due to the room exciting the bass which most of us are used to unless you owned a speaker like Quads and heard the lack of box and bass bloat, the open baffle speaker will be a learning experience. So there will be a learning curve, forget how your past box speakers sounded, your talking apples and oranges. Defusion is great for opening the sound up, but it can make things sound brights. Like Quads, the Spatials radiate sound backward. corner bass traps would work well. The front wall also has a panel behind the speakers.
The best box speakers I owned were the Dynaudio Confidence 5's. Greta bottom end, mids, and highs, the bottom end could shake the room, but they could not touch the open baffle bass of the M3's and other Spatial Audio speakers. They are very balanced top to bottom, and what I like about them is they can sound like whatever the recordings do, sometimes I say where the bottom-end, the next track it hitting me in the chest so unlike most box speakers the bass is not always the gorilla in the room. Just Listened to Boz Scaggs "What New" recording which he did jazz standards, and it was by far the best I ever heard it and I've heard it sound good but never this good from top to bottom, and the tone and color of each instrument, and his vocals right in the room, full, warm and natural.
Now with my room, floor, and carpeting I like the sound of my speakers without the spikes, I get more of everything without them, details, weight, tone, and color, with the spikes it is still there it will sound more ethereal, but I like the body and weight to the music, I use the spikes then I don't. I like something about both, but my ear always takes me back to no spikes. Plus they are easy to toe-in and move, lots of good articles on why spikes heard the sound more than help, I know on my Quads with spikes are would run out of the room, the sound because so titled the upper-mids and highs, sounded like a system where you turned the treble controls up. The same effect over many years using them under gear, if you like only clean detail then that is the way to achieve it, but that is not how real music sounds. Listen to a human voice when we speak it is not overly detailed, sharp, or airly. it laid back, had a throat and body to it and chest. A speaker designer gave me this tip once, listen to a human voice, 2nd advice from him, the bottom-end of reproduction supports all that we hear above it so the bottom-end impacts the mids- and the highs. Spatial's nail that.
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