OK friend. I did some more testing on gain settings. I am indeed hearing a very small amount of rounded out (increased?) bass with +6 relative to 0. I am not noticing anymore increase in bass to +12,but my h10 has less than than 10 hrs on it and bass is said to change with burn in.
I am definitely noticing the hole at 0 that you have described. This does indeed completely disappear with +6 and +12 is the same as +6.
It's not exactly a hole. Imagining a surround sound setup as analogy, at 0 gain I am getting voices through the centre, but all the other instruments are coming through right and left speakers that are placed right next to my ear (surround l and r, not front l and r) . In my test song, a piano in panning from right to left, but it completely skips the front l, r and centre channels, going directly front surround r to l. Drums are the same, pounding at me from surround l and r, completely bypassing the front l and r speakers. It's like the front l and r speakers are not included in the mix. With +6 gain the front l and r speakers are well used, and the piano pan indeed crosses right across my face, r to l. The drums are now coming out of the surround l and r and the front l and r. Everything is completely filled it. Front l and r speakers are being well used. +12 gain is the same to my ears as the +6 gain.
No idea what is going on with this lol...