Why have capacitors improved so much over the years?

@ieales  My friend has the PSAudio BHK 250 amp and said it sounds best with matched Mullard 6922 tubes.  Perfect matching is rare and maybe Kevin Deal goes for 5% and 10% for his premium and gold matched sets on Upscale Audio.   Maybe I've just chosen 5% and 10% matched 12AX7s, 6922s and other small gain tubes.   Output tubes are the most likely to alter gain and other characteristics during long term use.  That is why manual and/or self biasing is required for output tubes.

Illuminating indeed, @tomic601. Modjeski gave hour long talks at three of the Burning Amp Festivals (videos available on YouTube), the audiences filled with some other pro hi-fi designers (Nelson Pass, Mike Sanders of Quicksilver Audio), a few reviewers (Herb Reichert), and lots of amateur solder-slingers. A free education in basic (and not-so-basic) tube amplifier design.

I'm not debating matching tubes. Been doing it for decades. However, if you stick a matched quad in something like a CII or ST-70, bias it up, play it a while and then re-measure the tubes, they are no longer what they were and the ratios of the various parameters have all drifted slightly.

Quality tubes will likely still qualify as a matched quad, but 2x matched pairs are within the probability realm. As is a matched trio... and an orphan.