I'm a big Arcam fan, although I've never heard A90.
Arcam are traditionally fairly neutral, if matched with wrong speakers and source they can sound warm - others will probably disagree with this statement.
Compared to the A85 I think the A90 falls a bit short, a little uninteresting and dull, so I'm told. If you're prepared to bi-amp it with a P90 that'll be a different story.
Don't just go by star ratings, trust your own ear - make your own conclusion, and if you take into consideration the hefty discounts available, then it's a good, if not brilliant, amp.