I have not compared them head to head, but I have heard both and can say that the AP20 is on a class of its own. Being on this forum, you may have already come across my evaluation of this unit in another thread. I am an authorized Aurender deler, so take what I say below how you’d like.
The Aurender’s internal streamer in the AP20 is really quite good, almost at the level of a $13,500 Aurender N20. The DAC is also, very good, in par with something like an $8K T+A DAC 200 (I am also a T+A dealer). The amp stage is also really, really good, using Purifi modules and a very high quality power supply. In short the AP20 has a crazy low noise floor. I don’t think I’ve heard noise be so “black”. It’s a very dynamic sounding amp, oozing resolution and dynamics. Separates can do a better job driving some very difficult to drive speakers, but the AP20 can handle most speakers without breaking a sweat.
The Marantz pieces, as good as they are for the price, will be less coherent and less controlling. The streamer as well just doesn’t deliver the resolution and imaging that Aurender is capable of.
If you are already happy with the Marantz you may very well be blessed to settle on that combo and not invest the significant amount into an AP20. In this case ignorance can be bliss. But if you hear the AP20 and can afford it, you will likely not want to settle for less.