In the MP-1, starting from the rear, the 1st two positions are wired differentially within a tube and the 2 tubes are then in parallel. This is the bottom of the cascode. The 3rd tube is the top of the cascode. So the 1st two make the most difference.
In the MP-3, the 1st tube from the rear is the bottom of the cascode differential, and the 2nd tube is the top.
Generally speaking the noise importance of the tubes decreases as the signal progresses from the rear to the front of the unit, but that is not to say that the final tube is not sensitive to noise.
Generally we advise people to stay away from NOS in the phono section because of tube aging. In this case, not the NOS tubes, but the OEM tube, which is Chinese. Unless those tubes are new, they will be noisier than they should, so often NOS tubes replace them, and then the preamp gets accused of being noisy when really what is happening is the NOS tubes, while being quieter than the OEM tube (which might have gone down hill its own self), are still not quiet *enough*.
That is not to say that you can't find NOS tubes that *are* quiet, but they are rare these days.
In the MP-3, the 1st tube from the rear is the bottom of the cascode differential, and the 2nd tube is the top.
Generally speaking the noise importance of the tubes decreases as the signal progresses from the rear to the front of the unit, but that is not to say that the final tube is not sensitive to noise.
Generally we advise people to stay away from NOS in the phono section because of tube aging. In this case, not the NOS tubes, but the OEM tube, which is Chinese. Unless those tubes are new, they will be noisier than they should, so often NOS tubes replace them, and then the preamp gets accused of being noisy when really what is happening is the NOS tubes, while being quieter than the OEM tube (which might have gone down hill its own self), are still not quiet *enough*.
That is not to say that you can't find NOS tubes that *are* quiet, but they are rare these days.