I read almost everything here and I have too returned my iPad Air 2 to Apple because it only vibrated already at three bars. I have my second copy now and have just finnished same sound-test I did yesterday and the day before with my iPad Air 2 nr. 1. So my iPad Air 2 nr. 2 vibrates as well yes. I went also to my swedish biggest local stores (Siba and Elgiganten), and yes, they also vibrated when I played. But however, this new one didn't vibrated that much at three bars, standard from factory is six bars and there I noticed the vibrations. Yes I could feel the soundvibrations a bit on three bars as well and yes they felt through the Apple Smart Case as well, when you for example play a game. Or watch a Movie. When you go for a bit higher than six bars, maybe let say eight, I've noticed that it feels almost like my cars sub. So the stereosound is truly fantastic!
However, I will not return mine because of this. I can live with the #vibrategate. And especially for me comming from an iPad 2 (the brick), everything feels great. Also, if you take a good look what's inside it,
what kind of GPU it is, you too will accept these sound vibrations no matter how weird they are. Because this iPad beats the crap out of everything that's on the market and will keep doing that even next year.
I will keep mine because I don't want to wait for next year and next and next and the next after that to decide to buy that "perfect iPad", because nothing is perfect in this world. And this one will rock for a very long time and with it's spec that are as powerfull as a 2013 MacBook Pro.
So you can spend weeks to return it back or fourth or just enjoy this. Because it IS really awesome. And I love the Touch ID, the lightness it gives (compared to my iPad 2 brick). I was about to change it to iPad Mini 3, but when I saw the price between them, I would be an idiot to buy a Mini 3 when I can have a superpad for just little bit more money.
Cheerios!