Thanks again for the contributions. It is good to see how differently we approach the question.
First, let me briefly explain why a more detailed thought experiment on system optimisation might be useful. Visual art and music speak to a deeper level of happiness within us than, say, enjoying a lime sorbet or an episode of Game of Thrones ever can. That’s why I think listening to music in a concentrated manner at home goes beyond the concept of home entertainment. Just as art on the wall serves the purpose of decorating the room, but also deeply appeals to our intellect and emotions.
I see the audio hobby as a journey. Translated into a week- or month-long hike, this would mean that I decide where the stops might be and which place is the final destination. Compass and map help me to arrive at the desired places. If I then make detours, they are intentional and for pleasure or curiosity. Plus, I get to the final destination in the end. If, on the other hand, I often ask not only for the direction but even for the right destination along the way and everyone tells me something different, then that can also be interesting - probably also marked with dead ends, detours and circles though.
As Ben @all2ofme and Erno @Ermos said above - “Number/duration of grins for a given configuration” ingeniously covers it. That’s probably already the better compass than the one suggested below. My best attempt to cast everything into just one sentence would be “spending as many meaningful moments with music as possible”.
The question of optimisation criteria can be broken down into parts and thus made manageable. With which contents someone fills the framework described here is completely individual. However, I consider the framework transferable to all high-end journeys. The idea is adapted from the field of software design.
The whole thing starts with use cases. This is where it is decided how the system will be used. Everything from secluded indulgence to enjoying music together with friends can be captured. Possible use cases could be, for example:
Enjoying music . immersing oneself in the music . dancing . mirroring or triggering emotions . experiencing stories . venting . finding refuge . achieving healing . filling silence . facilitating routines . show off . nurture relationships . view with pride of ownership . evaluate . tinker
Part of the requirements result from the use cases. Another part of the requirements arise from the inclinations and aspirations of the users.
Aspirations could be:
(self-directedly) put together the ‘best’ system one can on a given budget . playback of recorded music as authentically as possible . spend as many meaningful moments with music as possible . immerse oneself as deeply as possible in music.
Inclinations would possibly include:
following a concert intellectually . intuitively perceiving music . leaving everyday life behind with music . tracking the sound of a recording . maintaining a music collection . experimenting with hi-fi.
If one adds the preferences for music reproduction, most of the requirements already become apparent. Requirements can be classified according to importance, e.g. essential (non-negotiable), important, nice to have or dispensable. This makes the decision easier in edge cases. An essential requirement will then not give way to one that is only ‘nice to have’. Here I have only listed examples of essential and important. Following Pete @PAR I distinguish between music presentation and sound. The difference can perhaps be illustrated with an example. Sound would describe what it sounds like when the drummer hits the bass drum - the reproduction of the moment when the mallet hits the skin. Music presentation in this context would mean how the drummer plays together with the band and how the bass line creates the groove.
Examples of music presentation requirements might be:
…essential . it makes music as opposed to it shines with sound . the artist’s intention is revealed . the music presentation does exactly what the particular performance was likely to be about - e.g. groove, stimulate, soothe, arouse emotions, stimulate the intellect, appeal to a deeper level within us, make the heart leap with joy . be able to listen deeply into the music . the music moves one internally . the music entices one to groove . easy intuitive access to the music as opposed to intellectual access.
…important . easily follow the individual musicians during the piece . the composition remains intact . the musicians play with each other and it becomes obvious how they play with each other . remain composed and accessible in complex passages . be taken out of everyday life . discover what the music is about - emotions? poetry? structures? time? . be able to rest in the music . be able to follow the individual plot lines . the inner context of the music is revealed . open up or unlock the music . the colouring of the music becomes immediately clear and is sustained e.g. elegiac, lyrical, joyful, sinister, angry . if given, be able to learn from the music . as short a time as possible until one can direct ones attention exclusively to the music and the body . as captivating and satisfying as possible . Reactions such as curiosity, being touched or forgetting time.
Examples of requirements for the sound:
…essential . accurate in timing . the colours of the music become clear when the recording allows it . gripping . engaging . long listening sessions are enjoyable . one looks forward to music listening and does it often . gets under the skin
…important . different recording scenarios remain intact . reproduces drive + punch where it is in the music . analogue . organic as opposed to sliced and diced . lyrical where it is lyrical as opposed to “here are the facts” . PA when it was PA . visceral playback . free of “fabric softener” . playback electronics largely disappear.
Requirements for the equipment
…essential . low-maintenance . failsafe . technical issues such as number of inputs or outputs are appropriate . fits in the room . WAF
…important . proper industrial design . tolerable resale values
For the actual optimisation, it is also interesting which yardsticks we use, how we find and select upgrades, and whether we want to make overriding considerations.
Yardsticks can be:
Concert . other live music . visit to recording studio . visit to fairs . listening tests at acquaintances or dealers . the heaviest possible power amplifier 
Selection of upgrades via:
trusted testimonials . experience of others who have gone before . press . forums . trial listening . own experience
Design concepts
Elegance . the whole before the detail . Focus on how mind and body react (emotions, reactions, thoughts) . Intuition says “yes” . Source first . Empiricism
Of course I don’t walk around with a checklist. Most of it is automatic and intuitive. After all, experience also plays a big role. However, this is an attempt to make what is going on explicit, repeatable and discussable. It’s a complicated compass, but one that works reliably, at least for me.
What are your thoughts on this?
