The Blackbird amps are indeed the next evolution of the Karna amps. Same circuit, but updated power supplies and simpler tube choices. All tubes are in current production with multiple manufacturers so there is plenty of choice. The only exceptions are the voltage regulator tubes, but there are literally thousands of them out there to buy for under $10 per tube. In the Raven, the damper diodes are no longer in production, but the same is true. Many to buy and they last for years.
As for the coupling caps, yes I have heard most of the great ones. I prefer a custom designed transformer. To me, the transformer has basically no sound. Not if they are custom designed for the circuit. The caps all sound very different from each other, plus they take at least 20 hours to run in, with subtle changes for 100 hours. The transformers sound the same from the first minute. I was on the cap side of the ancient cap vs. transformer debate for years. Then I got custom designed transformers.... Now I don't use coupling caps in any of my builds.....