Believe it or not, the same thing happened to me when I upgrade from my PortaPros to my AD700.
I was initial disappointed at first, claiming they sounded too similar, or not much of an upgrade. But after time, I really started to notice and appretiate those differences.
Me, as a potential buyer of the AD900, likes headphones with a large soundstage. But apparently, I didn't really know what that sounded like, even though I own the AD700... until recently. So I decided to to some tests yesterday. Some AB tests. I collected my 9850 IEM's and my PortaPros to go head-to-head against the AD700. I wanted to see for myself what soundstage really was, and what it sounded like.
The best way I found to tell a difference between the two, is listen to the headphone with more soundstage first. For us both, that would be the AD700. Listen to nice, expansive segments of songs that you enjoy. Now, switch headphones and play that same segment at about the same volume? What do you notice? For me, there was a
significant difference between my AD700 and all my other headphones. My AD700 felt like the music surrounded me, and enveloped me
into the music. It was more of a "listener-to-music" interaction, which I thoroughly enjoyed. When I switched to my other headphones, especially my PortaPros, I instantly felt like the music was cramped together inside, or within close proximity, to my head. It almost felt claustrophobic. And I didn't
feel the music as good.
Thus, after these tests, I have finally decided that I
really love a nice, 3D and spatial sound. Which perfectly justifies me upgrading to the AD900.