If you have $10K and are considering a Luxman, I cannot buy used on this one since I went and demoed them (will buy new) but for someone open to a used purchase.
https://www.usaudiomart.com/details/649568734-luxman-m-900u-and-c-900u-as-new/images/2474255/
In my opinion the difference with the m900u and c900u combo is massive compared to the 509x integrated. Yes, I have heard all three in the same room with the same gear (that is why I will buy new).
Edit: The price is for each unit not for both.
https://www.usaudiomart.com/details/649568734-luxman-m-900u-and-c-900u-as-new/images/2474255/
In my opinion the difference with the m900u and c900u combo is massive compared to the 509x integrated. Yes, I have heard all three in the same room with the same gear (that is why I will buy new).
Edit: The price is for each unit not for both.