Here's a bit of mathematics:
1000000.0 Ohm resistor at 25.0 °C within 20.0 Hz to 20000.0 Hz frequency band will have :
Noise Spectral Density = 1.283185e-7 V/√Hz or 128.3185 nV/√Hz
Noise within desired bandwidth = 1.813790e-5 V or 18.1379 uV
Dynamic range re 1V RMS = 94.8 dB (thanks to http://www.audio-perfection.com for maths).
This shows that a 1M resistor by itself has about a 95dB noise figure relative to a typical guitar. This is without any other electronics. Beyond that, the following fet or opamp will have a noise voltage, and a further noise voltage due to its noise current, which will be directly proportional to the input resistance.
An H9 has a dynamic range of about 92dB, including all electronics, so it is pretty good.
Obviously we could easily put a 20M resistor at the front end, but the noise performance would be reduced proportionally. It is a tradeoff.
Our feeling was that with most guitars there is little to be gained by increasing the input resistance above its current value. In fact, some people find it too bright see our FAQ Q: The H9 alters my tone, adding brightness to the signal. Why?.
Some pedals use a noise gate to reduce the percieved noise. We are considering this, but are concerned about "breathing" effects and cut-off of effect tails.
So, it's all a trade-off. If you can hear a difference and are prepared to put up with a bit more noise, by all means use the preamp of your choice.
