Maximum Upgrade Potential

Thanks for all of the thoughtful insights.  It seems a better choice to save some dough and plan on getting a new amp instead of spending $1000 on tubes.  I've been chatting with Fritz of Fritz Speakers about a pair of Carbon 7 SE Mk 2s.  If the deal goes through I'll lick my financial wounds for a while and use them as a basis to select a new amp.

Musical tastes certainly evolve and refine over time.  Twenty years ago I was listening to lots of organ and symphonies, primarily on CD.  Now, it's leaning toward quartets and early instrumental and vocal music, mostly on vinyl.  The Slayer CDs rarely make an appearance in the home system.

The thing is you can always make things sound different, and you may be seduced into thinking that different is always better. It can certainly be exciting to "hear things you've never heard before" but do it enough times and you realize it's different, but not necessarily better.

I believe that improvements over the last 20+ years, and still ongoing today, differ depending on the type of amplifier. Solid state has changed significantly, as pointed out by @ghdprentice using Pass as an example. However, changes in SET amplifiers (300B, etc), for example, has changed much less. Yes, there have been improvements in capacitors and transformers, but essentially the sound of a 300B amp today is not significantly different than one 20 years ago.