I have news to tell:
I was allowed to test the K2 tube mics at home for three days.
For a start, one of them was faulty! The dealer might have wanted to cheat me or something, because one of the shipped mics was visibly second class and when you set the polar pattern with the knob on the power supply, I could hear scratching noises.
The other mic was OK, and it had a very impressive sound. Its sound is very rich and detailed, but to me it's a bit short on trebles. On the other hand the sound has a strong 'vintage' character, one that's really different from the guitar's sound. I didn't like it first, but eventually I got to like it, because it's a nice, round, smooth, quality sound. The pattern knob on the power supply is very practical and professionally done. You can fine-tune the sound any time easily while you're playing.
I discovered that with the Tascam I can use the remaining one K2's sound spread out on two channels (stereo) for direct sound in cardioid setting, and the two inner ones' sound for background. The Tascam's MS (mid-side) decoder helped, and in the end I mixed down the whole on two channels and got a wonderfully rich sound with a lot of trebles.
After the test I told the dealer that I wanted to try the NT2000, too.
So now they are with me, they are real beauties, and definitely more practical than the tube ones. The mics themselves are much bigger than the K2s, almost a kilo each! They have a very nicely done surface-mount inside (made in Australia), a whole lot of condensers and a processor. The K2s fussy power supply is missing, so the carrying case is much more practically sized.
The test starts tonight and I hope there won't be a problem. I expect them to have a more open sound with more trebles.
As you suggested I've been looking for a tube preamp to improve the sound. I haven't bought anything yet, but I spotted a reasonably priced little Alesis tube preamp with 48V phantom, which could be used before an amplifier input. It looks good, but I've no real idea about its quality. I need two for the two channels and I'm afraid I can't afford anything more expensive than that.
The Tascam DR-40 is really great! It records with 24 bit/96 kbps, and if you use all of its possibilities, especially the 4-channel recording and the MS decoder options, you can be creative, and greatly improve the already excellent sound quality. Using the room's acoustic environment, mic positioning, the number of used mics, and maybe the type of mics, plus different stereo arrangements, the possibilities are endless indeed.
I'm considering keeping one K2 tube for main mic and having a NT2000 FET for supplement and using a tube preamp on each.
They would make a very nice pair, complement each other's character and weaknesses.
Music to hear, why hear'st thou music sadly?
Sweets with sweets war not, joy delights in joy...
William Shakespeare, Sonnet 8