Sound signature definitely changes as you change the volume and output power. I listen to the uncapped ZX707 on high gain at about 80-115 out of 120, depending on the IEM or headphone I'm using, what track I'm listening to, and what mood I'm in. If I go below 80, everything changes. Little to no bass, not resolving, details are not there, everything is bland, not engaging, not exciting.
Maybe this is why you hate your 707 so much. Maybe you're listening at low volume? Which is also why I asked what IEMs or headphones you're using, and you never answered.
It's why I've been saying in this thread that the capped version is not worth buying, especially at $900 USD. It's why I said Sony is dumb af nerfing the 707 in western countries and even some Asian countries.
I don't need to spend over $1,200 and buy a DAP I don't want, just so I can compare it for you. I already know the WM1AM2 has better sound quality than the ZX707, from a few of the users' comments here and elsewhere. Also on some foreign language reviews on YouTube. It should sound better, as it's twice the price of the 707.
https://www.head-fi.org/threads/new-sony-nw-zx700.966466/page-49#post-17401708
https://www.head-fi.org/threads/new-sony-nw-zx700.966466/page-58#post-17411085
https://www.head-fi.org/threads/new-sony-nw-zx700.966466/page-103#post-17508245
I know the difference between AM2 and 707 isn't close to as extreme as you've been claiming. They're not assumptions, as you've claimed. I have nothing to prove to you or anyone. You can show your proof you have both the uncapped AM2 or 707. Or not. I couldn't care less to be honest.
I bought the 707 for its smaller, lighter, more portable size. The good battery life. The newer, faster SoC. The newer, better Android version. The high build quality. And of course the cheaper price (imported from Japan) compared to other DAPs of this quality/tier. I'm very happy and satisfied with the sound quality of the 707 and don't feel the need to spend 2x the money to get maybe 20% increase in SQ at best, while sacrificing the other qualities I've listed.