Pros: Special mids, good depth, very comfortable
Cons: Not for bass heads, soft clamping force, tricky to get a fitting jack for recabling
I found each version has their own unique sonic characters. So it's more to your preference. But it took me 3 hours of sitting down and thinking hard, before I finally made the purchase of the Dog Pads version. Here's why...
The 840 version has a more forward and brighter sonic, thus making the midrange (especially for vocals) very special. However it fell short in the bass section. There is bass, but just adequate in amount, which to the broader audience translates into lacking bass or thin. The Dog Pads version has an improved bass response. I noticed improvements in bass body and punch. However the overall sonic is darker than the 840 version. An inevitable trade off, I presume.
The midrange is still special in a darker way. The depth is very good, instrument placement is clearly noticable. Layering is also good, I didn't notice any sound bleed to another. Comfort with the Dog Pads is better than 840 pads for a long listening session. Isolation is great and leak is also minimal. However, it is still not a bass head headphones. The Mad Dog is also not a forgiving headphone, so mind your music's recording quality.
The Mad Dog with Dog Pads will still benefit from amping. For me, I prefer a warm sounding solid state amp paired with a smooth analog sounding DAC. It did not perform as well on tube amps (tried on Crack and Pan Am).
That's all, folks!