I have the Superlux HD-668Bs headphones, not the HD-681s, and I took a calculated risk that these pads would fit. I can confirm that they fit perfectly! The HD-668Bs use the same size cans with a slightly differently design if you look.
From what I can tell and from the feel, I think they have used memory foam to make the ear pads, but I'm not 100% sure. Whatever type of foam they have used it's very good quality and quite firm. It's not the cheap, soft, weak type of foam with no strength; it holds its shape against the clamping force of the headphones and is very supportive. The velour is good quality as well.
My headphones are now a lot more comfortable to wear and I can have them on for hours without discomfort. They don't sweat-up like the supplied leatherette ones because the pads can breathe and wicks any moisture away.
I just wish these pads would have come as standard on my Superlux HD-668B Headphones in the first place; they are miles better.
Well worth the money and highly recommended.