The Fenestrated Sheepskin pads are a great level-up of both the comfort and sound of the HD6XXs, they're a little deeper in sound profile in a very positive way. Thoroughly recommended.
My HD6XX pads were really old and due for replacement. I decided to go with the fenestrated sheepskin and I couldn't be happier. I find that the headphones definitely sound a little darker in a very pleasant change, in my opinion. I was a little nervous about changing the sound profile too much but I wouldn't hesitate to go with these. And they're comfy!
I had tried the replacements they have on Amazon and they drastically changed the sound quality. Everything had sounded muddied and was a significant drop in quality. I will only be buying these through from now on. May be a little more but well worth it.
I bought these for my hd 5xx and there is no inner foam separating the driver from my ear. The outer padding is fine, but the original separator foam wore out and has holes and this did not come with any replacement.
I got a sheepskin one from amazon and that had a fabric separator sewn in which was great, but that ear pad wasn't as comfortable as the original.
This was just depressing.
The Fenestrated Sheep Skin's padding is much softer than the included earpads. They are not as structurally firm so that means if you have issues with your ear lobes being compressed, I am not sure I would recommend these because they can compress significantly. That said, they are comfortable and give a nice even seal around the ears. I have not done any sound comparisons, mostly because I use my headphones for work and voice rather than music listening.