I'm nobody's fanboy, but I do like the Air 2 more than my Air 1. They're both Space Grey, but I know I'm using the Air when pages take time to reload. I wasn't expecting a ton of difference, but that is big.
There's nothing wrong with a speaker having resonance. There's nothing special about the Air 2 being more resonant than the Air. It just is, but the Air has a resonate chassis as well. You won't escape that with either of these devices. I tend to stream movies and music to a TV or Bluetooth device. I only use the iPad's speakers for quick news videos, so while, it existed on my Air 1, it was something I never noticed. My wife uses the speakers on her Air to listen to music in the office. I've the resonance caused either tablet to vibrate on a desktop, then it would be an issue for me. It doesn't, so it isn't.