Why have capacitors improved so much over the years?

+1 roxy54!

It’s market driven. Who cared about capacitors 20 or 30 years ago. People like us who tweek and upgrade electronics have found that capacitors make a difference in sound quality. Like changing tubes and interconnects.

Engineers who designed professional gear 40 and 50 years ago cared. Back then I knew some then 'old timers' who were aware. Can't comment on earlier, but no doubt some were aware.

I had a tube 'guru' tweak my Citation II with better resistors, caps & wire. Same cct & old tubes, but OMG.

jfd11, hmmm, you have to tell us the name or it's like it never happened. 

Redwood, I bought most of mine from Surplus Sales of Nebraska, when I didn't make my own.

At the Surplus Sales website, click on 'capacitors' in the left column, then on 'air gap variable', then go to the 'quad' section. There are some with 4 sections, each of which maxes at 460pf, for about 1.9nF or 0.0019 uF when paralleled. With judicious design, one can create an RIAA network with values in that range.

Warning: they're big and expensive. But it's the best sound available.