That's Why It's Called A 'System'
Think about a chorus; The Robert Shaw Chorale, for example. Or maybe, to be a little more grandiose, the three hundred sixty voice Mormon Tabernacle Choir. Or go for broke, and think of all of the massed choruses of the Mahler Symphony #8 the "Symphony of a Thousand" in a full production.
Now, think about one single voice among all those others and, finally, imagine that the chorus becomes silent while the single voice continues.
Now you know all about hi-fi.
A stereo system of any kind, good, bad, or indifferent is, in a number of important ways, both the single voice and the choir at the same time.
Musically, that's obvious: Whatever the music is, regardless of its kind or the number of players involved, or the kind and number of instruments and performers, it's always the same system that plays it, and it's always the degree to which you can accurately hear and differentiate all of them together, and each from all the rest that's your basis for judging the quality of the system.
Non-musically, it's also true; the proof being that you can't listen to any "in-use" part of your system without listening to all of the other "in-use" parts of the system at the same time. That means that if you're playing the CD player of a system consisting of three sources (a CD player, a record player, and a streaming device), each playing through cables to a preamp, which is playing through more cables to an amplifier or a pair of "monoblock" amplifiers, playing through speaker cables to a pair of speakers, into a room that's furnished and decorated in a particular way and occupied by a particular number of people, the only things that won't affect the sound of what you hear are your record player, your streaming device, and their associated cables. All else yes, even including the number of people in the room will contribute to the sound of what you hear.
That's why a system is called a "system": Like a choir, it's a multiplicity of "voices" all contributing to and creating by their presence, their absence, and their degree of either or both a single musical experience. You simply can't listen to any single component or cable all by itself, with nothing else hooked to it. Neither can you listen to an entire system without a room to listen in. And, while your listening room and its acoustics certainly do affect what you hear, if there's no sound present, they cease to matter. It's the sum of everything that's important, not any one single thing.
Every one of the voices in the choir does make its own contribution to the whole but, importantly, that contribution may not always be positive. What if one voice keeps singing when all of the others have stopped? Or if one singer goes off-key or off tempo? Or starts to sing another song entirely?
With stereo systems, it's the same thing: Any one element of the total System including both the audio components and their acoustical environment can create negative changes that can affect the entire listening experience: Hum and noise, distortion, resonance effects, and problems of frequency response in the electronics; Microphonics, triboelectric effects, and even just poor, reversed, or dirty connections in the cables; and poorly designed, poorly placed, or poorly chosen speakers (think, for example, of electrostatics or other low-sensitivity speakers driven by classic SET [single-ended triode] amplifiers) can all negatively affect the sound played into the listening room.
And the room's acoustics, including its size, shape, furnishings, presence or absence of plants, people, and acoustic treatments, can result in clearly audible frequency response limitations, colorations, and anomalies which, coupled with echo and resonance effects, can make even a genuinely great audio system sound absolutely awful.
Even worse, all of any negative effects are cumulative: Problems originating in any one element of the System will be added to those from any element before it, and carried on through all of the System elements following it, to become a part of the total sum of what you hear.
Really; it is a system, and not just any one or group of its elements that you're listening to.
So what does that all mean? Rejoice; your system might be better than you think it is; there may just be some discordant voices along the way that need to be hunted down and silenced.
Opportunities for improving what you hear from your system may be present at any point, from your source, through your speakers, to the walls and furnishings of the room that you listen in. To find them, start from the very beginning, making sure that everything particularly your source components is properly set up and resonance-free; that all connections are tight and recently cleaned (that includes tubes and their sockets); that all cables marked for channel, polarity, or direction are properly installed; and that your speakers are in-phase with each other and are properly positioned.
Getting speaker positioning right, because it provides the interface where the sound and the listening room meet, may be the single most important thing you can do to ensure that your system will sound as good as it possibly can. The first thing to remember in doing it is that despite the fact that many audiophiles tend to keep a tape measure in their listening room for just that purpose.
THERE IS NO ONE DISTANCE BETWEEN SPEAKERS THAT IS PERFECT FOR ALL ROOMS, AND MAINTAINING PERFECT SYMMETRY BETWEEN THE SPEAKERS, THE SIDE AND BACK WALLS, AND THE LISTENER IS NO GUARANTEE OF PERFECT PLACEMENT.
Properly placing your speakers can take many hours of listening and experimentation. The easiest way that I've found for doing it is to use one of the commercially available "test and burn-in" discs that provides an "out-of-phase" test signal. Using it; put your speakers in initial positions near to where you think they might sound and "image" best. Then, sitting in your likely favorite listening position, listen to the out-of-phase portion of the disc. How does it sound? Does it image? If it does, you've got it wrong. First move one of the speakers and re-listen.
Then move the other and listen again.
Keep on moving the speakers, one at a time, in whatever direction makes the image worse. When (and if) you finally reach the point where the image disappears entirely and the sound seems to be coming at you from all directions, with no apparent source at all, you're finished. Now play some music in phase and you'll find that you have pin-point imaging and a perfect soundstage. This won't be possible in every room, with every system (and, especially, with every piece of recorded music), but the method works perfectly, and the worse you can get the image in the out-of-phase mode, the better your system will image in-phase.
Even if your system seems to be working perfectly, there's always the opportunity for improvement, and every improvement will affect the sound of the entire system.
Do what you can. Then put on some tunes, sit back, close your eyes, and...