Is it common for tubes to outweigh the cost of the amp or preamp?

@samiamnot- Not exactly, "serial numbers"(though some use the term), as that would be specific to just one tube. Typically, date codes indicate a week, month, year and sometimes a location/plant, for every tube in that time frame’s run. Your seller can probably provide more data, for your specific tubes. https://www.tubemuseum.org/SearchResults.asp?Cat=24 6922s are dual triode(two separate circuit/channel) tubes, like most other, commonly used for audio, miniatures & octals. Basically, two tubes in one. The closer the measurements(hence, "gain"), between two separate, paired tubes, the closer your channel balance will be, when listening. Regarding tubes in a push/pull circuit; one wants the individual triodes balanced, to achieve the most accurate(least distorted) sine wave(signal) reproduction.

@tomcy6 - Also of note- The silver shields all had an internal, stamped date code. All the early, grey shields, had screened, external date codes. I can attest to the(slight) brightness of the silver shielded Siemens. They’re not bad, just not CCa’s. I’m certain; they would sound beautiful, in a lot of systems. Still have a great pair of middle Sixties E188CC’s(silver), just in case.

When I bought my grey shield/plate, early Sixties, Siemens, they were just over $250 a pair(average). My CDP takes six, in it’s analog section. The player’s original cost was just under $5000, so I wasn’t upside down. The diff was remarkable, so- I bought six more, for spares. After about ten years, I thought I might compare the spares to the used. No apparent/appreciable difference (boy, was I glad!). I’m hoping the CDP will outlive me. You apparently enjoy the clean, wide open, extended sound of the CCa’s. If you’re ever in the market for 6SN7 types, the bottom gettered, 1940’s era, Sylvanias and Ken-Rads, offer the same presentation and are very close to the performance of such as the Tung-Sol round plate, or Sylvania 6SN7W(tall bottle/metal base). Much less expensive than them, too. 

@samiamnot- KUDOS, on your experimentation and learning experience. Now- simply enjoy the fruit of those (listening pleasure) and ignore the noise of the naysayers/cheapskates.