Sorry, don`t agree at all! Maybe you don`t have right tubes to get HD-681 depth and layering or maybe something`s wrong with your HD-681/setup/modding but saying HD-681 sounds 2d & almost one layer is like saying pigs can fly. Sorry, my ears & opinion and I bet many agree with it. Not trying to say that EVO is crap but even if there is difference, it`s tiny and that doesn`t make old HD-681 2d with one layer. But I`m glad that you like your EVO, enjoy it ;)
That`s exactly what I`m enjoying on my HD-681 mkII ;)
I use 6n23p-ev in my Project Sunrise II, it's not expensive but sounds very good and have good depth. There's nothing wrong with my HD681 mod or my setup, i have used more than 4 HD681: 2 for me and some modded for my friend and i must say, i know it very well. Saying red 681 has only one layer maybe too extreme, with well recorded song, it have quite layers to its sound but if the red 681 have 2 layers than the EVO has at least 4, the difference is not tiny at all, pls dont' judge something when you haven't heard it.