I think I'm going all digital with my Boss GT-1000Core. (TL;dr)

[Sedaxel]

When switching the HX series inputs from 1MOhm to anything else, the differences in volume pot behaviour are *incredibly* obvious, pretty much drastical even. With each and every passive guitar.

I can't tell for sure, obviously. All I can say is my Helix LT was not good in the input section, as I already said sometimes. It made something to the signal which I didn't like.

That said, I didn't notice excessive differences in tone from 500k up.

When I have some time, I'll try to experiment with a 10M pre I have, to see if the volume put makes something weird. As for now, I've read lots and lots of docs related to impedances in audio, and the only thing I could think that might affect the volume pot is just its relationship with input impedances, since the pot is just another impedance (variable). Never ever read anything about input impedances that would cause problems with guitar pot. This is the first time. It's always related to frequency transference.

 

[Sascha Franck]

I can't tell for sure, obviously. All I can say is my Helix LT was not good in the input section, as I already said sometimes. It made something to the signal which I didn't like.

It also depends a lot on your volume and tone pot circuits (treble bleed yes/no, which cap value, 50s wiring, etc.), different cables will make yet another difference at different impedances, too. But still, regardless of what guitar and cable I'm using, the differences are incredibly noticeable - the most on my volume pots.
Now, as I don't have any 2MOhm input circuits to try with, I can't tell whether things are in fact getting not noticeable anymore at higher impedances, but given how much of a difference there is even between some values sort of close to each other, I can't imagine going to 2MOhm won't be noticeable. Sure, possibly way less of a difference, but the volume pot behaviour will very likely still be changing quite a bit.

 

[JCW308]

Nice! I'm still 100% GT-1000 (full board) into FOH for all gigs for acoustic, electric and bass. No matter what I try, I just can't find a suitable replacement.