Nico,
I believe there were changes/improvements and in A/B between the A5 & A5.5 you could hear a improved difference. I can not say specifically what changes were made, sorry. I would not buy the A5.5 specifically for the DAC but there are a lot of positive reviews on the A5.5 and demand more money than the A5.
I currently have the Ayre hooked up but really miss the A5.5 in my system although I will feel the same way after I hook up the A5.5 and wanting the Ayre back. The A5.5 has loads and loads of juice, no problems in that regard.
In regards to looking at the A5, you can buy these for less money but I would shoot for the A308 instead if you can find one for a good price ($1,600), they were made in the UK and better suited for bi-wiring. The A5 can have a buzzing sound out of the amp that has been reported often and made in Taiwan.
With either one I have been told and read that a power cord and speaker wire can make big changes. Something I have yet to try.