Very interesting... I've been switching back and forth between +10 and +18 (it's kind of a pain, having to flip 4 switches on the bottom of the unit; I wish they could've put a single gain switch on the front, just like the input selector) and they definitely sound different. The sound is a lot fuller and weightier on +18, in more ways than one.
Firstly, bass is increased by a fair amount. There seems to be added warmth and depth as well. Main vocals sound fuller and chestier. Background singers sound like they're actually singing in full voice, whereas on +10 it sounds more like they're whispering.
Secondly, soundstage is a lot more full and layered, though not bigger per se. In fact, possibly due to the weightier sound, everything sounds more upfront and immediate. On +10 the soundstage was flat - like there's a giant wall of speakers 15 feet away, playing at you. They are a fair distance away from you, but everything is the same distance away. On +18 there is a lot more layering and depth to the soundstage. It's like you're listening to a live band with the lead singer in front of you, just a few feet away, guitars on either side of you, some instruments right behind the vocalist, other instruments much further back, background singers right behind you. In some ways it's more intimate, but more 3D and immersive.
I don't know if +18 sounds "better" than +10 in an absolute sense, but they are different. +10 has a light, airy, laid back sound. +18 is very engaging, dynamic and immersive. Are you actually "listening" to music, or just playing music in the background while you're working or relaxing? +18 is great for the former but +10 is better suited to the latter.
It's not all positive, however. While the amp is completely silent at 0 and +10 (although it does amplify any noise that's in the system if you turn up the volume), there is a very noticeable hiss at +18 and +20. The hiss is there regardless of volume, although it does increase somewhat if you turn up the volume. Not sure if this is due to the LM4652 or the amp design itself. It's too much work to open the thing up and swap in the stock opamp to test.
Also, there seems to be less clarity and more grain on the +18 setting.