I find that preamp tubes can be an important part to get the best out of an amp. For example you'll see people complain about the sound of a Dual Rectifier being buzzy, flabby low end, shrill high end, weak cleans, noisy, no dynamics, etc. Now this me be caused by many things, but because many Recto's are shipped with Chinese pre- and poweramp tubes it might be the most common source of these troubles (and the fact that people turn the gain up to ten).
I now only have writting some advice on 12AX7 / ECC83 tubes. Soon I'll add a piece about 12AT7 / ECC81 and 12AU7 / ECC82 tubes too. Because in many cases these tubes offer the right solution, especially if your amp needs to have some more headroom (see article on the Triaxis) or control over the breakup of the preamp.
Current production preamp tubes
Let us first get one thing straight. There are only a
few factories in the world that produce preamp tubes:
* Sovtek in Russia
* Svetlana in Russia
* JJ in the Czech Rep.
* Ei in Yugoslavia
* Sino in China
All current production preamp tubes come from these factories. So
Mesa/Boogie, Marshall, Ruby or Groove Tubes don't make preamp tubes. The
same thing accounts for power tubes as well. For one exception, which is
that Groove Tubes is now producing a 6L6 tube in the
USA.
Many amp companies, like Boogie and Bogner, design
their products using Chinese
preamp tubes. The only reason I can think of why
they do this, is if the amp still sounds ok with these tubes, it can't
get any worse. In my ears the Chinese preamp tubes are the worst
I've
ever heard. If you are a high gain freak that
loves piercing and buzzy (!) sounding highs and clarity isn't your biggest
concern, then buy them right away. But if you are looking for tone, decent
gain, reliability, dynamic and non buzzing sound then please leave them
alone. In my opinion almost any tube is an upgrade from a Chinese preamp
tube.
The Chinese tube is easy to identify. On the right you see a picture of it. The highlighted part is something I've only seen in Chinese tubes. So that's the easiest way to identify them. Another thing you will see is that it has 2 insulation spacers at the top. These are two horizontal plates directly above each other, they are pressed against the glass to stay in place.
The other tubes that are used a lot come from Sovtek in Russia. They've produced different tubes over the years and have lately produced their best tubes so far, the 12AX7LPS and the 12AX7EH (sold as Electro Harmonix ). The noise is lower and the tone is much better, but they still have a bright top end that may give amps a buzzy top in their lead stages, also the clean sound isn't as toneful as most other tubes. I regard these as o.k. repair tubes and people who are on a lower budget, but there are certainly better alternatives for aprox the same price if you ask me.
The last tube I can not really recommend is the
Ei/Yugo ECC83, which is too bad. The
sound of these tubes is simply amazing for high gain, their top end has
something special which give them an amazing good sound. But there is one
major drawback, they go instantly microphonic and are thus useless for
normal use in a guitar amp. That's really too bad because it's one of the
best sounding tubes for high gain imo. Maybe buying some special selected
Ei/Yugo's and saving them for recordings only, is a good idea, otherwise I
see them as useless. 
Besides the previous mentioned tube companies, there are still two factories making preamp tubes: (Tesla/)JJ and Svetlana . We can keep it short on the Svetlana as 99% of production goes directly to Marshall. According to various sources these tubes are not stable and the quality isn't as high as expected. And then we have the JJ's left. These are built in the former Tesla plant and are modelled after the Telefunken ECC83, one of the best tubes ever.. The sound of the JJ's is warmer which is great if you want to get rid of the buzzy sound in many of todays amps. They have a very deep low end and distinct mids and sound great both for high gain and clean.
Together with a friend we took out the Sovtek LPS in his Peavey 5150II and replaced them with JJ's, the clean sounded much better, more dynamic and better tone, and the lead tone was more full, less harsh. Some people complain that these tubes are too dark, which could be in some cases. But I highly recommend them for people who want more tone and less buzz from their modern high gain amps that use Sovtek, EH or Chinese preamp tubes.
ps. I read that the current
production of the preamp tubes in China have been improved. Also Ei/Yugo
have been working on new tubes, which seem to be a bit better.
New
Old Stock preamp tubes
But
there is of course a large collection of NOS tubes available. Some are
really expensive and rare, others are widely available and affordable. In
many cases NOS tubes sound better then most current production tubes
simply because they used better materials and higher standards back
then.
If you are looking for the real Fender type
twang then I highly recommend using GE 5751's, they have a bright top end
and late break up. Using these in Fenders or in the clean stages of
Boogies will give you a fresh and clear open sound with lots of dynamics.
In high gain stages they give you more control over the break up which can
be great to get that pushing/breaking up sound. The sound is pretty raw
and bold, don't expect silky smooth endless sustain, so these are not what
I recommend for solo's but for rhythm guitars they are great. But in high
gain situations they tend to produce more noise then some others. Still
these are great as main input tube and for all other applications to get a
clean and clear sound with plenty of detail and lots of
headroom.
For a warmer sound then I can
suggest you check out tubes from Philips . 
They have a great
warm sound, deep lows, low noise, good gain. The USA made JAN spec tubes
tend to have more top end then the European (Miniwatts). I personally
prefer the European ones made in Holland or imported from Mullard. They
have a very warm sound that's great for cleans and smooth leads.
If you want a true rock/blues tube then RFT 's are definitely something you should check out. These East German made tubes were also sold as Telefunken, Tronal and other companies and are known to produce a sound that is very close to Mullards. They are extremely low noise, plenty of gain, deep lows and a nice warm top end. Their breakup is great and they are very reliable and low in microphonics.
But when it comes to bang-for-a-buck, nothing beats original 70's
Tesla tubes. They were made by and for Telefunken
on their machinery and
they are identical to Telefunkens. I have a nice stash of E83CC (12ax7)
tubes from Tesla, they are simply amazing. The detail is by far the best
I've heard, frequency spectrum is wide and balanced, quality is high and
tubes are all close when tested. Original Telefunkens will go for a lot of
money, but these Tesla can be found at affordable prices. They bring the
best out of any amp. I've tried it in my whole setup, in a Bogner Triple
Giant and a Peavey 5150 II for example and have sold many to very happy
customers. These are the ones I'll be working with the next
years.
These are the tubes I recommend if you want to
go NOS and don't want to spend too much cash. Of course there are many
other fine NOS tubes out there that sound amazing like Telefunken,
Mullard, Brimar, Siemens and Sylvania, but I haven't been able to test
them all, or they are soo expensive that I can't really recommend using
them unless you have Grade A gear and the money to spend.
For more information on tubes check the following links: