668B is 56 ohm.
My laptop drives it to ear splitting levels. Even my MP3 player can drive it to very loud volumes. So you concern with Creative whatever is not a concern anymore. =D
Here's the discussion thread:
http://www.head-fi.org/forum/thread/485039/superlux-hd668b-are-they-really-that-good-or-just-a-cheap-copycat
Rockgrotto forum has a tonne of discussion on modding the 681.
From what I read, 681 has a slight V-shape equalizer sound: and thus more 'fun' to listen to. And for that reason, it has more room to mod its treble & bass. The 668B is more neutral, some say clinical and less fun.
But to me, the 668B sounds absolutely fantastic for vocals. It's clear & detailed, the bass a bit boomy (personal preference: I dislike heavy bass). Timbre/tone quality/colour/whatever of instruments and voices can be distinguished easily enough. Separation of instruments are also pretty good.
After some very simple blu-tac & cotton, the bass is reduced & tightened up.
I've posted the links to the mods on the link above. The mods for 681 works for 668B too.
It sounds like I'm over-hyping the 668B, and I probably am. But for the price, you just can't go wrong with it. It might not be HD650/K702/DT880 good, but it's good enough for me. It's so cheap it doesn't really matter if you eventually dislike it after buying.
