There are a lot of interesting answers in this thread.
In considering what's most influential in our listening rooms, which I believe to be important, I want to step back to ask a question with a wider view, specifically because listening to music is more than a technical exercise created with 'equipment'. To admit my own bias here, I want a presentation of music that takes my breath away, affecting me deeply at an emotional level.
In my not so humble opinion, I would like to ask whether, based upon the [predicted] variety of answers above, we, and the OP, might benefit from an examination, and perhaps refinement of the question Celo originally posed.
Here's one / the reason the answers vary so widely, and it may hint at which of the previous posts are more likely to be on the mark: every listening room differs; every Combination (caps used for emphasis because italics is not available) of components interacts differently than another combination. Further, every combination of components sounds different in a Different Listening Space. Finally, the same components' sound can differ Wildly in the same listening space depending upon their Physical Position in the room and the Attention that has been invested (yes, Invested) in their Complimentary Selection and equally, their Setup.
Let's rephrase / refine / and divide the question this way (for example, to accommodate those who have WAF to consider, though that's just one example of the reason to refine the question).
What is the aim of the listener?
I've heard three quarter million dollar systems, one that sounds like a wall of Marshall rock amplifiers, a second that sounds not much at all like Any form of music I've ever heard, and the third that sounds pretty good but that certainly, for the investment, should sound one Heck of a lot better. The first plays at ear-splitting levels yet doesn't image and can't render classical music with depth, nuance or delicacy. Is that important to the owner? No, and it's a dedicated listening room not subjected to WAF, highly --if not well-- treated for sound quality. The second was at a stereo store, set up by the manufacturer's setup specialist, and the third has a custom designed listening room in a dedicated Building. Hmm. Something's questionable about this situation, but what?
Some rooms can be no-holds-barred, well bankrolled and sound less than satisfying [to my ears], yet fully satisfy their owners. Price point does not determine quality: in the room mentioned above, everything was of a very high quality (cables cost more than my entire system and I'm in no way jealous of the sound; just the opposite) and those components were selected with considerable care to work together by an intelligent person. Clearly self-knowledge is diagnostic in this case; the listener gets what he / she wants. Whether it pleases me or others is irrelevant.
What about the listening room that has to share space and / or function with other family activities? What is most influential in the quality of listening experience in a non-dedicated listening room? That's arguably a different question, due to the shared / multiples application(s) of the space, and as such begs the posing of a different question, based upon its unique combination of applications --the listener's biases and expectations.
What are your Listening Priorities, Preferences and Expectations? How well, and how honestly, have you asked this of yourself?
Does the system you assemble 'Fit' your listening preferences? If you listen exclusively to E. Power Biggs on the Busch Reisinger organ, mini monitors aren't likely for you. Chamber Orchestra wasn't well presented on the expensive system mentioned above.
A friend who sets up systems had a client who fell in love when he heard my friend's system. He bought every single component my friend had, set it up. Hated the sound. Sold it all and got out of audio. The deal is that Those components, In my Friend's Room, set up with his His Experience (and highly educated ear) sounded Wonderful. The components were Top Flight, so it wasn't the gear that was at fault.
I had the unusual good fortune to have an expert evaluate my system In My Listening Room. Jim Smith --of Get Better Sound fame-- did a what he calls Room Play, measuring the room's technical characteristics and adjusting the position of the components and the room based upon his reference selection of music.
My system is arguably middle-range high end gear. I had worked with Jim's book and got the system sounding better than ever before. His work, however, took my system far beyond the best I could do, resulting in what I found a 'Musical' listening experience. The music, the gear, And the room never sounded better.
It is, in my experience, safe to say that no Single factor or component can said authoritatively to be Most Important in determining sound quality of Any room. However, no room, component or system will ever sound its best without attention to the whole.
A wholistic approach to audio is, in my opinion, what causes so much disappointment in many people's approach to creating a listening environment that fully matches their needs, expectations or hopes. What's sad about this is that what's behind this thread --asking a revealing question-- is so seldom 'tuned' to the listener's needs. But that, too, is a Catch 22: like getting your first job, you have to have experience. But how do you get your first job?
The degree to which a listener knows, admits and works with his / her listening preferences IS determinitive of outcome. In That sense, it is the listener him- or her-self who is Most Important in achieving the satisfaction that high end audio can offer. In this sense, Every choice, selection and matching of components, (careful (and often tedious)) positioning of them in a listening room, vibration isolation, tweaks and continuous curiosity brought to focus on how sound might be improved are subservient to a listener's willingness to be honest about the preferences, biases and desires they bring a priori to their listening chair.
No room treatments, component choice or amount spent can outweigh the 'Listening Life Unexamined': start there. To paraphrase Eric Clapton, 'Before you accuse [your system/ listening room, etc], take a look at your self'. And then pose, or if necessary rephrase, the question of what you want from your listening experience. That will lead you first, to a refined understanding of your priorities and second, to a substantially improved listening experience, perhaps taking some considerable time. But it is an investment of those hours in fulfillment. Can you put a price on that?
It will help you raise the bar.
