Q for any Luxman L-509X owners

This is from manual.
Separate switch (SEPARATE) Separates the pre-amplifier and main-amplifier each other. off : Uses this unit as a normal pre-main amplifier. on : Feeds external signals from the MAIN IN terminal on the rear panel to the main-amplifier section. • Every pressing of this switch toggles the separate on and off. The separate indicator lights up when the separate switch is on. When the separate switch is set to on, the volume control of this unit cannot adjust the volume of the speakers connected to this unit. Volume adjustment shall be performed at the input device side such as the control amplifier connected to the MAIN IN terminal. Entry of direct output into the MAIN IN terminal from a CD player or other devices that cannot adjust sound volume constantly provides a full power state and accordingly results in the risk of speaker damage. For such input devices, be sure to use a control amplifier equipped with the sound volume adjustment function as a relay, generate sound through the speakers with volume lowered, and adjust the volume to your favorite level. When arranging the wiring, be sure to turn off the power of this unit

Separate indicator (SEPARATE) Lights up when the separate switch is on. The subsonic, monaural and loudness can be toggled only from the accessory remote control (RA-17).

And manual for remote.
Separate switch (SEPARATE) Separates the pre-amplifier and main-amplifier each other. off : Uses this unit as a normal pre-main amplifier. on : Feeds external signals from the MAIN IN terminal on the rear panel to the main-amplifier section. • Holding down this switch for 1 second toggles the separate switch on and off. The separate indicator lights up when the separate switch is on.

Thanks for further details on this. I'll have to check tonight if it works by holding down the button for 1 second.. I don't think I've done a "long press" on the button before but that would make sense!

slightly off topic, what speakers do you guys use with it, and how does the amp sound?

I'm using Klipsch La Scala IIs and in my opinion they're a great match with the 509X. 

The LS IIs already have pretty stunning dynamics and smooth mids, as well as solid control of the bottom end. The L-509X complements all of those characteristics with passages that seem to literally move with dynamic expression. 

Big, spacious, expressive sound... I think you have to hear it for yourself, I wouldn't know what you're benchmarking it against, but for me the La Scala IIs are a pretty great fit for the 509X.

Here's a video I took of the "1812 Overture" with the 509X and La Scala IIs:

https://youtu.be/sB_wChJ0RlI