Why are older tubes so much sought after

There are a couple of issues at work.

First, many of the NOS tubes were made in factories that no longer exist, using machines long since gone. For tubes commonly used in audio equipment there are few to none made in the US, Britain or Germany anymore. Some feel the modern factories in Russia, China and other countries just don't turn out quite the same product.

Second, there is the rarity factor. As NOS tubes are put into use they don't get replaced. With any "luxury" item (and fine audio equipment does fit in that category), an increasingly dwindling supply of irreplaceable parts certainly lends a certain cache to the old stuff even if one argues the new production is just as good.

Consider yourself lucky that you like the stock tubes best as it just makes your life easier and less expensive.

Just one more comment from the peanut gallery. The Soviets in particular continued to use tubes in a lot of their military equipment long after the US military had moved much of their equipment to solid state. (In fact, they felt vacuum tubes were more resistant to the EMF effects of a nuclear bomb and gave them an advantage in the survivability of their electronics.)

One can get into the endless loop debate of whose military had stricter QC standards for electronics manufacturers (and the US has had its share of equipment procurement scandals over the years) but the military angle alone does not explain the differences in tubes.

Sirspeedy wrote: "Very tight tolerences!"

Just a comment on a minor point. Tight tolerances has nothing to directly do with sound qualities. It only means whatever the attributes of the design, the finished product consistently meets specifications.

That could just as easily mean that the tube is consistently not suitable for audio.

I've noted there is a tendency by some to almost automatically ascribe good qualities to the old stuff. However, having lived in the 50s and 60s, I know that period of time had its share of poor and mediocre products. There is little doubt in my mind some tubes from that era are less than they could have been.