I had the same thought as you, however I own a wm1z and wondered if I could go cheaper and lighter with the wm1a without a compromise in sound quality. Especially with the custom fws the differences seem to become less and less, or so I thought. So I also asked some friendly hf-members who have both devices, wm1a & wm1z, and they said that there still is a difference in sq and that they liked the wm1z better, even with the same custom fws. For me that was enough to stay with the wm1z and not try the wm1a.
I can't say anything about the wm1a as I didn't hear it, but the wm1z is such an amazing sounding player, it just makes everything sound super sweet and incredibly musical. I never heard a source, desktop or portable, that sounds that good. I also own a zx300 with custom fw and the wm1z is the much better sounding player, so much that I still always carry the heavy thing wherever I go.
But as you had the wm1a and you were happy with it, just go for it. The wm1z is much heavier, so not really good for on-the-go and of course double the price.
Best thing is of course to buy both and decide for yourself, the prices you mentioned are really fair (especially the wm1z at 1200€), so you may not even make a loss if you sell the one you like less. And hey..... if you try them both, please let me know what you think, as I am still curious