Well, as Nick mentioned above, the H9 must have a signal to noise ratio of 90 – 100 dB to pass our quality control routines. What this means is that if your guitar signal comes in loud enough that it's close to clipping when you play then the noise floor is 90 – 100 dB quieter than this. And that's a big difference. The problem is that if you have a quiet guitar which you are plugging straight into the H9 you may have a signal that peaks at say -60 dB. That means that you have only a 30 to 40 dB separation of your guitar from the noise floor. If you put this into a high gain amp and say you boost your guitar by 30 dB to compensate, then you are boosting both your guitar and the noise floor by 30 dB. If you instead put a boost pedal before the H9 and boost it there by 30 dB, then you're going to have 60 to 70 dB separation of your guitar from the noise floor, and if you raise the signal a bit more in your amp, you're going to be less likely to hear any hiss. And that's effectively what you're doing when you run the H9 in your effects loop. Your amp is boosting the signal to more or less line level before it goes into the H9. So, you're getting a better separation of your guitar from the noise floor. If you want the same quality of sound when you put your H9 in front of your amp, then you'll want to put a boost pedal before it (when using quiet guitars).