It isn’t the knob that turning so well....it is the physical built of the RK50. However, Sony customized it with Sony specific designs, and so they are calling it RK501.
The knob is simply screwed onto the connecting shaft of the attenuators which by itself is turning so smooth and nice. As I mentioned, the RK50 by ALPs is very revered in attenuators for many audiophiles equipments. In my own experiences, I have it built on into my KG-T2 for my Stax system, and I know how it sound in comparison to other stepped attenuators I had built on the other amps such as Carbon or Grounded Grid. The BHSE and GSX Amps also have the RK50 as an optional upgrade
Now, if you asked, could you buy the attenuators alone for that “tits turning sensation”, yes you can, just buy RK50, and any knobs.....only that it cost $1000+ per unit. *The DMP-Z1* is expensive for many reasons. After all the reasons are comprehended, the DMP-Z1 is actually cheap for the price