Sick of closed headphones wanna try some open ones. the Superlux HD668B + some decent new ear pads for it is 50 buck and the athad500x is 80 bucks right now. I know it will be better than the hd668B but is it that much better?
I have the 668B's with new pads and I love them.
From what I've heard the 668B's sound signature is very similar to that of the AD500x.
Sound Stage however goes hands down to the AD500x, so if you play games where positional audio is important the AD500x is probably superior.
However I have heard ccomplaints about the comfort of the AD500x's, specifically about the pads being quite thin so you ears touch the plastic housing of the driver. I solved this problem with my 668B's by sticking some cotton wool between the earpad and the housing, but I have very large ears so YMMV.
TL;DR. Yes for the soundstage if they are comfortable to wear. If you don't care about soundstage so much, probably not.
What about the HD681 EVO?
They've got reasonable sound quality for the price and they come with both leather and swappable velour pads in the box.
Dont wanna get closed ones.
They aren't closed-back.
I had both the 668B and the EVO in the past and have found that they aren't really all that different sounding. The EVO is a little more V- shaped in sound. I prefer the 668B mainly b/c of the physical features like comfort and materials. My EVO creaks a bit when you bend it around and the materials feel pretty cheap. The camping pressure is also skull crushingly strong although I did stretch them out on the box it came in for a few days to help with that.
I don't have the velour pads, but I did do a sock mod (among other stuff) with them. 
I would get 668b for sure man :)