It's not the same to use the HX One with a clean amp as with a boosted amp; that's where you really see if it introduces noise. (You can have the same hiss problem with overdrive or distortion pedals). I recently bought an HX One and haven't yet been able to test if it introduces noise with a boosted amp (I have a Blueguitar, a Marshall JVM 205H, and a Plexi). I had a similar issue a while ago with an early HX Stomp, which was very noisy (hissing), but didn't produce any hiss with an M5. I sold the HX Stomp, and later tried a new one I borrowed, and it didn't introduce any noise either. I think it might be a manufacturing defect in some units, or in the early models, as happened with the HX Stomp.