I think I unexpectantly stumbled upon a pad-endgame set up with the Pro 900s. I got some Edition 10 leather pads and decided "why not?"
Well, it is crazy how different headphones sound with different/the right pads. It's honestly shocking just how much the acoustic environment matters vs. the drivers (MrSpeakers probably is the best example of this phenomena).
So I had to fiddle around a lot to fit the over-sized Ethiopian Leather Edition 10 pads onto the Pro 900s, but I got them working and on tight at least for now. It is crazy; it sounds like a completely different headphone.
-Dramatically larger soundstage; completely open feel, even more open than the Mad Dogs (have not heard the Alpha Dogs yet). I feel that the S-Logic is finally working as intended because I can clearly hear instruments completely around me, as if I am flying overhead a symphony playing live. This matches David Mahler's impressions of the Edition 10 in the flagship thread. I am truly shocked how much the pads were a factor here.
-Dramatically reduced bass. This could be a deal breaker for most since that's what the Pro 900s were the best at. The extra distance from the driver very noticeably reduces the mid-bass slam. I am somewhat mixed about this since the Pro 900s were my fun headphone for mainstream music, but now just entered a new world of accurate, spacious sounding closed headphones. They are still colored, but they don't do bass the same when the driver was half the length away that it is now.
-No hint of harsh highs/sibilant treble. The sound now is not fatiguing in any way. It is great! I can crank the volume up for a long time without feeling that ear aching sharpness/piercing treble.
-Less isolation. Not sure what the deal is with this one, or why the isolation was reduced, maybe due to the pads being perforated/holed for heat reduction? The Pro 900s feel very open and not closed in anymore which is great.
I don't know how I feel about the reduced bass though, it's a bummer. I am really enjoying the new sound though for now. I truly feel that the Pro 900s sound like completely different headphones now. Something spooky about the Edition 10 pads.