Ok, I finished the full TopPop mods to the RX900 last night (full Dynamat (Extreme), stuffing the housings, removing the felt and the diffuser), and have been listening and comparing them to the RX700 all morning. I also burned them in all this week, so I now have over 100 hours on them.
I gotta say, I was hoping for more. The bass is more controlled and bleeds less into the mids, but overall, these mods didn't do what I was hoping. I was hoping they would help overtake the RX700. They did not. I still prefer the forward presentation and clarity of the 700. The 900 (although better now) sounds bloated, stuffy and echoey in comparison. Don't get me wrong, its sounds way more expensive than it is, but RX700 just sounds better.
I thought the muddiness and echo sound would be eliminated by the dynamat, and I dynamtted the entire back of the cup and surrounding the driver plus the driver itself, but it wasn't. I think its just the nature of this phone. The bass quality and quantity are now the same on both phones, which is plenty let me tell you, but not too much, at least for me. Some people may not want this much bass.
At this point, I still like the RX900, especially for its price. But it's not as crisp and clear as the RX700. It does have slightly more fullness and presence, which is nice. I actually wish I had a phone that has the forwardness and clarity of the 700 and the fullness of the 900. But to me, the bloated tones of the 900 distract me when I know I could be listening to the 700 instead.
So the 700 is still the winner in my book. Best $30 ever spent!