Got the 25 Alu's today. I think they have no sibilance like Amperior did in the upper vocals/lower treble. Bass is slightly less than amperior but excellent punchy bass :) definitely a keeper!
But the cable is a bit too long for convenient portable use. Anyone know of decent cables that are shorter for a reasonable price? (third party cable welcome)
Just joined the HD-25s Aluminium club! I'm also coming from the Amperior and I think you sum up the subtle differences pretty well. The sound signatures are of course similar, but the slightly hot treble and occasional harshness of the Amperior is smoothed out; they seem a touch clearer, fuller and more rounded overall, with the incredible tightness and accuracy I've come to expect from Senn's HD25 range. I had my old pair of HD-25-1 II for many years and I can see the HD-25 Alu being my go-to portable for a long time to come.
Dunno whether you're still looking for an aftermarket cable but I would also recommend the Oyaide HPC-HD25 - quality cable. First thing I did after getting these headphones was get one in. The stock cable is definitely too long and the right-angle jack of the Oyaide is a bonus too.