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.

Thank goodness Tvad provided such a perfect response, now I don't have to reply other than thank you.

This is an interesting film on the production of Mullard Valves at the Blackburn plant.

www.techtubevalves.com/about_us/index.php

Click on vintage film reels

Good answer Tvad! There are less than a handful of companies now producing tubes that compare with NOS in quality of materials, care in manufacture and sonic performance. Here's some info and an idea of what that quality and care cost in today's dollars:
(http://www.kraudio.com click on "tubes")
(http://www.elusivedisc.com)
(http://www.euroaudioteam.com The only company making quality small signal tubes now)
(http://www.westernelectric.com)
(http://thetubestore.com)