I don't say it, because i've heard it somewhere. I have them here, all of them. The bass of the 681-B is more controlled, better kickbass, better texture. and better Highs. The Highs on the 681 are very harsh and the bass sounds (to me) bloated and not defined, similar to the EVOs bass. Everything is just better balanced in the B Version, similar to the modded Version of 681.
I know housing etc. has a big infuence BUT i too have the Samson SR850 and the Presonus HD7. They too look similiar to the Superlux and i opened them . . . guess what i found? Superlux Logo on the drivers ;D They sound exactly like the 681 / 681B I think, the ISK is just another rebranding and i think its sounds better than the 681 because maybe its a rebranded 681-B and not 681. 681-B sounds better, and there are not much people, who know the B Version. Most people know the standard red version, thats why they think, its a different driver/brand. My 681B almost sounds like my Shure SRH440, almost identical. Thats how big the difference between the 681 models is.
Look at the Samson and Presonus:
. . . they're al rebranded superluxes. I tested and opened every one of them.
That's just your opinion, i see many more people prefer the 681 to 681B, in my country, the superlux 681 is 40$, 681B is 28$ but the 681B is not selling very well, people like the 681 better and some of my friend even order from amazon to get the 681as backup when it was discontinue in my country. Having heard both, i think the different is small, the 681B have tighter, more control bass, a little clearer but the 681 go more deep in sub bass, fuller body, sound smoother and more enjoyable.
I agree that the samson are clone of 668B, they look exactly the same exact the headband and sound the same ( i have both) but the presonus HD7 does sound different than the 681 (some people confirmed this). About ISK580 vs 681, you should look carefully again, they look very different.