I focused on this PSS-A50 product because I read many interesting articles from "pjd", on his site : http://sandsoftwaresound.net/yamaha-pss-a50-look-inside/
When focusing on the PCB, I noticed that only pins 1 and 2 (Left channel : DAC L+ and DAC L-) of the Yamaha YMW830-V Processor and tone generator (IC101) are wired on the PCB, not pins 5 and 6 (DAC R- and DAC R+). It is known that this product is "mono" (one speaker, and despite the 3.5mm headphones jack is stereo, the sound outputs "mono").
This product is declined in three variants : PSS-E30, PSS-F30 and PSS-A50). If we consider the two first variants are entries and design to cost, it can be acceptable to have a mono product.

I was wondering if the "high" variant PSS-A50 (it has a MIDI feature that the two others don't have) would have the same limitations ("mono" sound), and especially if the tone samples in the 16Mbits flash memory are mono or stereo.
My first proof of concept will consist in recreating a stereo audio path from the YMW830-V pins 1,3 (left) and 5,6 (right). For that, I would re-create a stereo asymmetrical low pass filter in order to feed the audio signal in any audio "Line-in". The audio input of my PC in my case.
At this step, I did not retro-engineered the PCB to recreate the schematics. Just wanted to check if the sound outputs stereo or not.

The Block Diagram of the audio section looks like this :
BlockDiagram_PSS
In order to get the stereo signals again, I wired the DAC L+/DAC L- and DAC R+/DAC R- outputs of the SWLL to an external active Low Pass filter with a basic LM324 and a few resistors/capacitors (same schematics as for PSR-F50... visible in the service manual of the PSR-F50)
BlockDiagram_PSS_Stereo_LineOut
I wired the DAC L+/DAC L- and DAC R+/DAC R- outputs of the SWLL to a 2.54mm header : PSS-A50 PCB small
and connected this header to my Low Pass filter implemented on a breadboard : Stereo Breadboard small
The result is visible and audible in this video : https://www.youtube.com/watch?v=cwDBfVab3vc