not much older woof, they pretty much launched together. the a1 is simply the model up.
if going via denon link be nothing in it one vs the other I imagine. the difference is really with the dacs, power supply analog stages. and yeah if I remember its pretty much best of best category in that regard. its full balanced design made to go for the denon avp and poa combination. it was a large large step up in price though. and again like the 4010 its possibly end of model cycle, perhaps denon will update it perhaps they wont. who knows.