Thanks for all the responses, I see the can is open and the worms are everywhere! I want to answer the questions asked as best I can, but I feel I should start here:
Are you generally dissatisfied with many things in your life, not just your audio system? If so, then I think you may be looking for the solution in the wrong place. A good analyst might cost less than a new preamp or amp.
Perhaps I over-egged the pudding with my first post, but we're not at that level of angst! I forget sometimes that I'm new here and that other members can only judge based on the words I type, and that none of you actually know me (I'm a long-time member on a local forum and have met virtually everybody, so they're used to me). To that end, rest assured that I'm well-adjusted, happily married, live a good life that I generally enjoy, I laugh a lot and don't get too stressed for the most part. I'm not even a natural upgraditis sufferer or box-swapper (I've owned 3 cars in my entire life, for example). Hifi-related comments should not be construed as signifying any deeper need, but it's a point well-made, Larry, so thank you for asking!
Right then, despite the impression I may have given to the contrary, I really do believe that my dissatisfaction is caused by a specific set of sonic issues, and that these problems can be solved. However, it seems clear that my current method of upgrading isn't working for me, as my approach seems to take me too far down one path at the expense of others. I always pushed for resolution and clarity, each upgrade had to give me more details, and on the face of it that seems sensible. However, "clarity" for me was always a function of mid and treble openness, and as a result I always ended up prioritising that over everything else. In my 20s I was that guy who was completely seduced by the sound of well-recorded female vocals and small-scale baroque music, and I built a system to maximise that. Some part of me thought that if it did that well, if it conveyed the purity of the human voice and accurate instrumental timbres then everything else would be good too. And to an extent it is, if everything is recorded *just so*.
I'm at work so I don't really have time to go into more detail in terms of what causes me to turn off, but at the most basic level if the recording is even remotely hot it's going to be difficult to enjoy on this system (which is indeed as per my sig, btw). If there's any kind of large-scale complexity that needs a solid underpinning, it's going to sound quite compromised. If the recording is warm (or even distantly-mic'ed) then it'll probably sound good, and I have lots of examples, but there are just too many recordings that I can't enjoy for me to be happy. Rock is very difficult (in the words of one non-audiophile friend: "It sounds tinny") but these days I don't listen to so much of that. I can't decide whether I don't listen because I don't want to or because it sounds bad. Jazz generally sounds good, but even early Miles Davis can be a turn-off because the tonal balance seems so tilted towards upper-mid hardness. Large-scale choral works can sound unnaturally spotlit and thin, as can orchestral music. Classical piano and organ music often lack scale, and I'm not even talking about pedal notes here.
For me, it seems to be a question of tonal balance, with upper-mid exaggerated at the expense of upper bass. I'll try to confirm this with room measurements in due course, but no matter what they say this is what I hear. The final kicker: I'm a pretty good organist, and have spent a lot of time playing, accompanying and listening to organ and choral forces. Trying not to compare my system to the live experience would be an exercise in futility, but I never expect to recreate that at home. I just need something that does it better, and that can provide a more even-handed approach across the board. I'd lose some resolution and clarity for that now.
Final thoughts: I'm still a music lover, please take my word for that! I'm a musician myself, I hear a lot of music in the course of my life, a lot of it live, and I consider music to be the greatest of mankind's inventions. It raises goosebumps and bring tears to my eyes, irrespective of the playback medium. I haven't fallen out of love with music, far from it.
Final final thought: I've heard lots of systems I like (and lots that I haven't) but the recurring theme is that systems I like have a certain broad brush "sweep" to them, systems I increasingly don't like are of the forensic sort. I used to favour "warts an' all" but I'm starting to think that I no longer want that. However, I don't want to go too far the other way in my search for gorgeousness of sound.
That's quite an epistle, thanks for reading if you've made it this far! I'll try to deal with more specifics if I get a chance later, but I'm really enjoying all the comments and views expressed here.