For me, being done is about both the music and the system. I think the music part is self-evident. It is the reason I got involved in audio in the first place.
On the system side, coming to terms with this question was much more challenging. There is little thought-leadership on this subject beyond the old mantra that the next significant performance upgrade is just around the corner (cable, isolation, CD player, power conditioning, etc), and that afterwards you will attain audio nirvana. I'm not saying that these do not improve system performance, in some cases they do. But they aren't the point, are they?
I tried a different approach:
1) I re-defined my system goals and objectives. I asked myself, what is being "true to the source"? Instead of the master recording, I re-defined this further up the production chain, to the intent of the performers, the intent of the conductor, and even the intent of the composer. I know how a great live performance makes me feel inside, and I know my reaction to a tepid performance. I don't have any idea, or really care about, what happened in the recording studio, or what the sound engineer intended. As my system makes me react more like I do to a good performance, it is "better". This is not the same thing as getting more detail, resolution, power, etc. And given this evaluation method, a cheap table radio or car stereo can be a great system.
2) To get the above, I re-defined how I view my audio system. I like to think of my system as similar to a musical instrument like a piano or violin or guitar. So rather than trying to continually "upgrade" my system, it simply is what it is. Like a piano is a Yamaha, or a Bosendorfer, or a Steinway. Is one piano better than the other? Or are they simply what they are, percussion instruments with different flavor of sound?
3) After experimenting with enough gear in my own system, and listening to other high-end systems at friends and dealers, I have enough context to realize that at some point it is no longer about getting better, it is already good enough. At some point, the improvements are marginal, even if the differences are there to be heard.
So, I still enjoy gear. There are a few things I will continue to do with my system. I want to add a mono cartridge to my turntable. I may do some mods to my phono stage. But I am pretty much done with the important stuff.
Questions / comments welcome.
Scott