From my experience the 509X is as neutral as it gets. Very smooth and crisp, never fatiguing. Only thing I thought it somewhat lacked in my setup was the mid-bass and bass. I demoed a Vitus RI-101 side by side and with the Vitus it felt like I added a sub in the room (nothing else was changed other than swapping amps). This was with the Martin Logan speakers I had at the time. Same thing with the Gryphon Diablo. Both are double the price, so the Luxman is a great value for the money.
Luxman tonality
I’ve seen many members describing the Luxman 509x and the m duo as warm thus colored. I’ve had these units and have never thought of them as bright or dark(warm). They are superbly neutral to me neither adding or subtracting. I find them to be very musical. A prospective buyer might pass on Luxman thinking that they won’t match up with many speakers. Of course they will. Neutral is neutral. It’s what we should want.