I.ve been using the ZX1 for the last few weeks along side a DX90. I got mocked a bit in the DX90 thread when I said that to me the ZX1 is up there if not better than the DX90.
Here's my reasons.
The DX90 is a great player at a great price, almost half the price of the ZX1. It drives all my iem's with no problem and it probably has slightly better detail and layering to the sound than the ZX1. However the ZX1 has something that I cant quite put my finger on that makes me use it more, its not the UI which by the way is far easier and more fluid to use than the DX90. There's just something about the sound that makes me want to use it more. To my ears it feels less clinical than the DX90, more musical, thats not to say the DX90 isnt musical because it is, its just not quite as smooth and lush as the ZX1. This becomes more apparent to me when pairing with the DAC section of the PHA-2, this adds a whole load of smoothness and extra detail to the ZX1.
My intention was to keep one and sell the other but I cant, I love both players but if I had to choose I would probably plump for the Sony.
I owned the DX100 a few months ago and although not able to compare directly, I would say that both the ZX1 and the DX90 come very close and in some ways surpass the DX100 both in terms of SQ and ease of use, although this is using fairly easy to drive
iem's, I know others prefer the DX100 for full size cans.