Law of Accelerating Returns

Sounds_real_audio - the math depends on if the 5% improvement is relative to a perfect standard (100%) or the current level of performance (90%). The total in one case is 95% and in the other it is only 94.5%!

I get the notion that incremental improvement in one aspect of the system can improve the overall sound of the system (and reveal other weaknesses, shortcomings or colorations). But once a system reaches a certain level, getting those incremental improvements usually does cost more money. (Let's leave aside low and no cost tweaks and set up issues for a moment and assume all that has been addressed).
I know this sounds like $=improvement but the reality is, the better phono cartridges within a given brand, to take one example, tend to be the more expensive ones. Is a ten grand cartridge a worthwhile investment? Perhaps, in a system with other issues already sorted. Is it going to materially improve a system with other needs? Probably not as much- or put another way, probably not as much as addressing the other shortcomings first.
So, one brings the entire system up to a level that arguably has no pronounced 'weak' link; sorts 'mains' power, room, set up, etc. Gear is top level, whatever that is, for the sake of argument. Now add that top tier phono cartridge. Will it be a noticeable improvement? Maybe. If properly matched with arm, tonally compatible with the rest of the system and the listener's bias, etc. To get a system to a level where all that makes sense costs time, effort AND money. And it is at that point that somebody could reasonably say, 'it may be better, but it's not really worth it to me to extract that last '--nth' from my stereo. And i'm not spending 10k on a god damn cartridge, no matter how good it is!"
So, I'm having a hard time accepting the idea that these incremental improvements - investing more to extract that last iota- don't cost more money. I think the reality is, they do. I also don't find % characterizations about improvement to be very helpful. It lends a false sense of being able to quantify and compare differences that are largely unquantifiable, though real.
If the message is, pay attention to all the details in set-up, placement, room, mains power and the like before going out and buying the next piece of expensive gear (with the expectation that your system is going to drastically improve as a result), I'm totally on board with that.
Have certain components made a dramatic difference in my system? Yes. But, the system was already pretty dialed in and everything else was at a pretty high level of performance, so I knew where my 'weak link' was.
Where have I seen the biggest difference in sound at this point? The pressings. I'm satisfied enough with what my system does in my current room that I've been spending most of my time on sourcing the best pressings of the music I like. And there, you can find dramatic differences in sonic result. From flat and lifeless to vivid and punchy and alive sounding. Unfortunately, many of these are old pressings that are known as better sounding, and finding them in unmolested condition is neither easy nor cheap. I suppose someone could have the same reaction there, too- I'm just not going to bother buying that XXX pressing, even if it is better. No way is a record worth $XXX dollars. And I get that too.

If you use live acoustic music as a reference point for 100% then it becomes reasonable to assign a percentage to mark progress toward that goal. It's admittedly a subjective measure. Percentages as a way to describe improvement have been common vernacular between audiophiles for a long time. Percentages are typically used to describe progress from the origin. But that approach doesn't account for the eye-popping improvement("accelerating returns") that is sometimes reported after making a small tweak near the finish line. The Zeno paradox is a thought experiment that helps account for this.

I guess I'm stuck because:
1.no reproduction system consistently sounds like real music; yes,
sometimes a convincing illusion on the right material, but listen to enough
different source material and you can hear the 'machinery at work'- the
illusion falls away, and you are left with a very good reproducing machine,
but one that cannot fool you all the time. I think that's the nature of the
beast. Gear is generally better today than it was 20 or 40 years ago, but I
can still hear the 'seams.'
2. we all (or better if I just speak for myself, "I") have different
ideas of what real should sound like. A little more bass. I wish that vocal
were a little more prominent. Why, when things get complex in the musical
program, does it sound congested? Some of this may be system-related,
and some, in my experience, is the source material. But, at bottom, I think
the ideal - DGarretson's 100%- may be different for each of us, and that is
going to dictate what each of us strives for, assuming an unlimited budget
and access to gear.
3. Most people don't have an unlimited budget and access to gear. We mix
and match using commercially available gear, with some tweaks, to try and
achieve that special 'synergy' that says 'real' to the person in control of the
system and the checkbook. This results in that endless merry-go-round of
equipment swapping, or the realization that last month's 'astounding
improvement' has paled by the time the credit card bill arrives.
4. DGarretson- it looks like you got off this merry-go-round by modding
commercially available equipment. I assume you did this for at least three
reasons: out of the box, the gear could benefit from an improvement that
the manufacturer didn't offer; cost-effectiveness, and subjective 'tuning' to
get the system to sound more 'real' by your lights. Most or at least many
audiophiles (and I'll include myself in this camp) aren't blessed with the skill
to start tearing down electronics and replacing the innards. So, we are left
with trying to find that elusive synergy among commercially available
'boxes' along with 'tweaking' in the easy ways- isolation, positioning, cable
changes, gear swaps, tube rolling, etc.
5. I guess my point is that the realization of that elusive 100% is a very
personal, subjective thing. And that once you are close, getting it to that
last 'nth' degree is neither easy or cheap.
6. For me, the midrange is always the killer. I can live with more limited
bandwidth or even limited dynamics, but if the midrange isn't grain-free,
vivid and 'in the room' it's going to sound 'reproduced' to me. Others may
conclude that bandwidth and soundstage are the markers for real.
7. So, where does that leave us? The 100% is at best an impossible goal. It
will never really sound like real music all the time. Do we give up? Most of
us here, are here because we aren't satisfied with 'good enough.' So, we
continue the 'quest' within the limits of time, energy, knowledge base,
access to gear and budget. But, and this is my hypothesis, not some
absolute statement, knowing your biases- recognizing what makes the
illusion better for you and where you can brook compromise- makes the
journey a whole lot easier. (And one's knowledge and one's preferences
may change over time too, which just adds to the length and difficulty of the
journey).
8. I have stepped off the merry-go-round, not because my system can't be
improved- it could benefit by better, deeper bass and a larger room to set a
bigger stage for the speakers (and probably by going to a larger set of
horns as well). But I am at a point where the differences in gear at this
point, for me, in my system and room, make less difference than the sound
of particular pressings and masterings. And, since the objective at the end
of all of this isn't just gear, but what the the musical result of the whole
system is, including the source material, I've found much happiness in
buying lots and lots of records, most of them old. This has, in turn, led to an
exploration of cleaning methods and their effectiveness, which is a whole
other subject.

Where have I seen the biggest difference in sound at this point? The pressings.
Whart

Whart nailed it. And if you take a look at his system, it makes even more sense.