Why are older tubes so much sought after

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)

Trelja- Just curious what 7199's you're using. I owned(and modded) an ST-70 for about 15 years and found the Sylvania and Phillips ECGs to be the best. Have you installed a NOS 5AR4 yet? Talk about another revelation! I used a Mullard in that position. Regarding the others' experiences with their un-named NOS tubes of unknown origin: (like everything else in this hobby) Your results will depend a lot on into what these tubes are plugged, their condition/quality, and who is doing the listening(many variables). The best NOS tubes just get out of the way of the music and add or subtract nothing, which is the definition of, "transparency", and you've removed one variable by using them. From there it becomes a matter of taste and/or what may be lacking in the rest of the gear.

Trelja- I gave my son my ST70 some years back for his birthday. He ended up with RCAs as Sylvanias weren't available when the ones in it bit the dust. Everything else in the system was different, but- They sounded like an excellent tube(especially the imaging and soundstage). My Cary monoblocks take 6550/KT88 output tubes(matched quads necessary), so- nothing NOS is actually an option regarding price. I'd have to sell a couple ounces of gold to purchase the EAT KT88 Diamonds right now, so- I'm using Svetlana 6550's(latest iteration, ST Petersbug plant) from Upscale Audio. I was really pleased(shocked/surprised) that they actually peformed as good as the NOS Phillips ECGs I had in the Carys. I tried some other current production tubes in them first, and couldn't get them out of the amps fast enough. Off the subject: If you want a quantum leap in the performance of the ST70- Upgrade the coupling caps to film/foil, and install a better power cord. I used a Zu Birth without having to modify the chassis in any way. Those were just two of the basic things(among a host of others) I did to mine that REALLY yielded major results as far as accuracy.

Trelja, Mrjstark and Larryi: Amen and Amen!!

Again- Info on some of the precious few companies manufacturing tubes comparable to NOS quality/transparency, and the prices they are commanding: (http://www.kraudio.com click on "tubes")
(http://www.elusivedisc.com/products.asp?dept=1681)
(http://www.euroaudioteam.com The only company making quality small signal tubes now-click on EAT)
(http://www.westernelectric.com)
(http://thetubestore.com/wesel.html) I have to agree with the "economics/supply and demand" scenario as to why more aren't taking the time and effort to produce tubes as good as those(granted- the select "Grails" and certain choice others) from the 40's through the 60's.

Hi Mechans- I've got six KenRad bottom getter 6SN7GTs to go with my six Sylvania VT-231 bottom getters for backup power amp tubes when the RPs(drivers) and Ws(phase splitters) run out. I just bought another pair of RPs out of Australia last week, and I've got one pair of backup W's. Below 250hz(10th order), my bass is handled via Hafler TransNova 9505. But the mid-bass speed that the W's provide is hard to beat. Really the biggest difference I notice between the Syl VT-231/KR GT and CTL/CHS combos is in soundstage depth. Other than that they are very close in timbre, tone, imaging, harmonic bloom, ambience recovery, frequency extention and(of course) all four iterations are the most transparent of the family. I can live with that, should my Grails fail. I dearly wish NOS 6550's or KT88's were still available in matched quads(or octets) for less than a king's ransom and first born male child(I could afford the second part). I'm saving my pennies for an octet of EAT Diamond KT88's.