I just traded my MDR 7550 for MA900 and I'm gonna get it tomorrow or Monday.
My expectation for MA900 is that I think I'll retain most of 7550's sound but with added front-back space to the sound. 7550 to me has really broad left-right space to thje sound but front-back is kinda shallow as expected of IEMs.
Hmm :/ There is a fair anount of difference between the two. The MA900 doesn't have quite as much a linear low end compared to the 7550. The 7550 has very tight bass extension that is quite flat and well extended, the MA900 has more of a mid-bass kind of warmth to it. I love both but I don't think they sound all that similar.