Massimo-
Sorry it took so long to reply. Scinx has said that he doesn't think the C1 is flat... and I would agree. In fact, in my first post I said:
"The C2000B is probably the most natural sounding while the C1000, the NT1 and the C1 all have more characteristic mid or high frequency peaks, which can be nice for certain clean guitar sounds."
Though in all fairness, I should not have mentioned the more peaky ones because your initial request was:
"I want something that going to capture the sound as close as possible. "
I just also got some good results from them, so I mentioned tham anyway. Some would say you shouldn't use use them... well, fine.
I also said that the condenser mikes are meant to be more accurate than dynamic mikes. I said this because condenser microphones, just because of how they are designed, are able to follow the soundwaves coming from an instrument (this is called transient response) more closely than dynamic mikes . This does not neccesarily mean that their frequency response is more accurate.
Yes, if you want flat, get a measurement mic (which is a mic made for the purpose of having a flat frequency response). But there are also many mikes that are relatively close to flat that are easily found in catalogs or stores.
The Shure KSM 32 is one of those. But it's probably out of your price range ($600).
I would agree that the sm57 and other dynamics can work on guitar amps, but you said you want to record something that sounds like it does coming out of your amp. So I told you what you want:
1. A condenser mic- this gives you more accurate transient response than dynamic
2. A mic with a very close to flat frequency response
3. Placement of the mic at a distance that allows for a more natural sounding pickup (too close is not representative of the actual speaker sound)
People might try to convince you that you're not getting an accurate sound with an sm57 because you're not pointing it the right way, but they're wrong. The sm57 is meant to have a very particular sound, good or bad. For one, the bass frequencies roll off starting at 200 Hz and there's a 6dB peak at 6 kHz. This mic warps reality for a very particular purpose.
Of course you might just think you want accurate recording of miced guitar. You might hear it recorded and understand why everybody seems to like that 6dB peak at 6 kHz. But I would suggest you get a fairly flat condenser mic, play around with it and see what you think of it.