I've listened for about 3 weeks with the Focus pads, and wanted to share my impressions.
I'm powering the 400S through the balanced outputs of a Pono Player. The cable is a Wireworld Pulse Nano that the manufacturer terminated for Pono balanced (2 3.5" plugs) on one end and the 400S on the other. The grills are off (which to my ears increased detail and ambiance without any penalties). The other source is an Ayre Codex. I listen mostly to jazz, rock, pop, and assorted acoustic music.
In this setup, the Focus pads shifted the headphone's tonal balance down (less treble, more bass), which I found pleasing at first; in general, I tend to prefer a darker sound. Later, I also realized that I was also hearing the midrange as significantly recessed, which made voices, guitars, brass and reed instruments, etc., sound pushed down in the mix and somewhat indistinct. The clear, natural, airy midrange is what makes this headphone special for me, and the Focus pads definitely made me enjoy it less.
Today, after replacing the stock velour pads, I was surprised by the difference. The beautiful midrange was back, the balance was brighter, and while there was less bass, it was tighter and better defined. Also, the stock pads brought back the upper treble harmonics, which make instruments like saxophones and guitars sound more realistic. Overall, I was surprised by how much I preferred the stock pads. Perhaps the Focus A would work better. But given that I spent $40 on these, had to deal with Hifiman's pokey shipping and customer service, and the badly designed pad mounting scheme, I may not find out.
