What exactly do you mean most apart? They sounded different than the rest of your superlux collection so you kept those?
Or different from the 681 line?
The normal 681 has the most representative sound, so to speak, of all 3 models. The other two, B and F, only dilute the qualities of the original, so to speak, so for me they weren't worth keeping. Hope that makes sense.
It usually has to do with the tonal balance of a headphone. For example a headphone with higher energy in the upper midrange and treble will generally be colder sounding.
While a headphone with more energy in the bass and midrange will tend to sound warmer.
In the case of 668B, relative to the normal 681 it has to do mostly with the treble. The 668B has a bit less treble emphasis and more prominent lower midrange. The end result being a bit warmer and more balanced sound.
Personally I prefer the 668B, but my music bias is towards the classical/jazz end of the spectrum.
But there are other people which prefer the 681 for it's hard hitting bass and forward treble, which goes very well for rock, metal, electronica and so on.
I can resonate with that but I cannot give you a definitive recommendation.
My music taste and expectations are different.
Also my 681's are quite heavily modded and have a filter to reduce the treble energy to the degree I've found pleasing.
So it ultimately depends on how you will find the treble of the 681.
If you can live with that and can EQ them to your taste, they'll probably make a better choice for your music.
The 668B are a safer bet, as they can be tamed completely from EQ, and they don't require much modding.
As for the pads, I've made all my purchases for Superlux and Samsons from thoman.de, and they also cary a wide variety of pads.
It would make sense to buy them from a shop that sells both, to save on shipping costs.