Maximum Upgrade Potential

You don’t. Because, even if you get the very best of today (I am an Audio Research fan). By the time you break it in and enjoy it for a few years, there will be a new model that is a step up.

Practically speaking. While if you love a certain companies sound. Then five years will get you incremental improvement with the next model… but ten years nets you a really big improvement. I find ten years… actually with any component is a really big difference worthy of investing in.

I owned Pass designed amps for nearly 40 years. And over thst time ten years was a huge difference. I recently heard a couple of them and the improvement in rhythm and pace over the last ten years has been remarkable.

FWIW, I think that for most 'ears' it is more about 'different' than it is about 'improvement'. Given a certain level of quality to begin with (high) IMHO most of what the manufacturers are doing with new products is tweaking a product and, perhaps, just moving the sound in a different direction. Yesterdays 'romantic' became todays resolution/clarity/dynamic range, but not, I think, because of any fundamental changes in design so much as popular expectation. In lesser products new iterations are not necessarily so constrained.  :-)

At over 20 years it don’t owe you much.  My tastes changed over the past 20 years, and if it’s in the budget I’d side with upgrading to another amp depending on your tastes.  There are some great amps out there from the likes of Octave, Audio Hungary, Audio Research, VAC, Unison Research, etc. so just read reviews and give yourself a well-deserved gift after 20+ years if something looks really interesting.  Prima Luna is good for sure, but there are higher levels of performance if you (and your budget) are up for it.  Tweaks and changing tubes will only get you so far.  Enjoy the journey!

I think, you answered your own question - you are ready to upgrade the amp. Cost is always a consideration, of course. I too would not be engaging in endless tube rolling after 20 years of owning a piece.

Without knowing anything about your taste, system and budget, it is hard to suggest any brands, but you didn't ask, anyway.

There is no question that people’s tastes change over time. They tend to work their way from the obvious parameters like bass and overall detail to imaging, then tonal balance, midrange bloom… etc. as they learn.

However, with well established high end companies… Audio Research, Pass, dCS, Conrad Johnson, MBL… etc. They are on a mission and incremental changes are not lateral at all… they are walking straight down a line towards their objective sound. This is not to say companies don’t falter a bit occasionally. ARC really tried to do what they did with solid state and they just could not do it and reverted back to all tubes. They have a well defined objective.

But successive releases of products, reduce the noise floor, improve the sound quality across the spectrum. Take ARC and Pass. One from the tube side and the other from solid state. Over the 90’s and 00’s they incrementally converged on reality… one from the warm but not as detailed side and one from the harsh solid state side. Converging on reality. 

Over the last twenty years, I spent a huge amount of time calibrating my listening skills, by attending hundreds of symphony concerts in the ideal 7th row center location, and acoustic jazz concerts… and listening to individual instruments. The same kind of things that top audio designers would do. I can hear them approaching and converging using different technology to do so.

So while the companies that have been around for a decade and are in lower tier high end may change this way and that. The companies that are serious long term players are moving in unison towards a very specific goal, with some individuality based on the technology they use, or which music type they want to sound the best.

By way of owning a decent amp(your case the PL Dialogue) for an extended period of time suggests the PL is competent at what a tube amp is supposed to do.

Tube amps aren’t’ doing anything different in those 20 years. It’s not an absolute must, buy you’ll just have to open the wallet wider depending on what you want.

There’s another level of WOW-possibly limited only by ca$h spent.

13 year PL user.

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.