I picked up the Focus Pads for my HE400 recently. Here are a few of my initial impressions compared to the velours:
The most dramatic change I noticed is in the 2khz-4khz range. There is a very nice lift in this upper mid and low treble area that really, really brings out guitars. I always loved the HE400 for jazz fusion, but I could never get into it for rock. The upper harmonic "lift" that gives guitars their attack and edge was missing. But bass, drums, keys and brass all sounded great. The focus pads really flatten out this area on the HE400, making them a great match for rock, metal, etc. Definitely a bit closer to neutral overall.
The entire low end continues to perform beautifully. The extra emphasis on the high mids/low treble tends to bury it in the mix a little more, but you don't really loose anything either... There is just more of the other stuff, so it's not quite as in your face on a flat mix for jazz/rock/metal, etc. It's still extremely present and weighty overall. On EDM, I haven't noticed any loss at all, in fact, I think the bass sounds better than ever before with the new pads. They just seem to obey the mix much better, making the headphone more transparent than it was with the old velours.
Comfort is in another league altogether. I never liked the velours. They were too hard and too scratchy. Weight aside, I would put the HE400 with the Focus Pads in the same league as my HD650's. They're not quite as comfy as the DT880 (nothing is), but it's in the league with the next best thing in every regard except weight. I also added a DT990 headband cover on the top, which helps tremendously in the comfort department.
Overall, the Focus Pads are a fantastic buy, in my opinion. If you're an HE400 owner who loves guitars and wants more comfort, they are an absolute must-buy.