So, correct me if I'm looking at this wrong, but here's what I see.
It looks like the focus should possibly be on registers 4, 5, and 6 because they seem to be what configures the three inputs (Mono, Stereo 1, and Stereo 2). It also looks like bits 0-4 consistently are what control the output gain on all three of those registers. Bits 5, 6, and 7 on register 4 are also what set the input mode configuration it looks like, and from what I can tell, we are using mode 6 by default.
Assuming that is true, we can look at bits 7 and 0-4 on register 5, the tech sheet says these configure the class-D amp gain (for modes 1, 3, 5 and 6) and the volume control gain. So, following the tech sheet, we could set bits 7 and 0-4 to a different value to bump the gain for any of the outputs listed - I think.
So, if we assume that is correct, that means that the stock value of 0x55 defines the class-D amp to +6 db, and the volume control gain to +8 db gain over zero for a total of +14 db gain. Following the same thinking, the 0x59 defines the class-D amp to +6 db and the volume control to +12 db gain over zero for a total of +18 db gain. This makes sense because the 0x59 is used on the speaker, and the 0x55 is used on the headset.
Now, since bit 7 on register 5 is set logic low, that means we are only using the +6 db gain level from the class-D amp, and since the volume control only runs up to +18 db, that means we are limited at +24 db gain overall - unless we try with logic high on bit 7 for the class-D amp. If we set that bit to logic high, it should bump the gain to the +12 gain level. This would add an extra bump in gain on top of however bits 0-4 are configured.
Assuming all this is correct, if we wanted to keep the same overall gain of +18, but use the +12 db gain range of the class-D amp to go from +24 of maximum range to +30, we would configure it to 0xD3 (UNTESTED THEORY). That should be +12 db class-d, +6 db volume control, for a total of +18 db. In theory, this would sound the exact same as stock, but would allow us more head room if we maxed out the volume control gain at +18 db and still didn't have the volume we wanted.
Now that I've rambled all that (which may or may not be correct), it seems like it shouldn't be TOO hard to figure out where the regular old headset speaker is hiding in all that hex numbering, then bumping the gain for it however we need to.
EDIT - The part I'm also not sure about is why are there 3 config files, and which one is normally in use? Also, does anyone know if the changes to the text file are "real time", or do they require a reboot each time? I might be willing to experiment a little if I knew that.