Guys, I have some details regarding the FS-5000 but I am not sure where to put it. Some of it likely also applies to the FS-5500 series (but I cannot confirm as I don’t own that machine). Some info also applies to at least the FS-4600F (about the disk ROM). So here goes:
1. As is written in the wiki already, the machine has 2 volume controls. 1 main volume control slider and another turn knob volume control for the headphones. Additional information is that the headphone circuitry is behind the main volume control (so turning down the main volume will also impact the headphone output).
2. This machine, and also the FS-4600F allow to format disks in 40 tracks per inch mode (this besides the more common 80 tracks per inch mode). The machine allows this for both single sided as well as double sided. Thes details is not present in the wiki of the machine (and maybe should be added to some more general wiki page as well?). Also I formatted a disk double sided, single track (the BDOS calls it just 2 sided on call format) and copied MSXDOS.SYS and COMMAND.COM onto the disk (from within Nextor booted from a Carnivore2). I then inserted the disk into a FS-A1ST and it booted from the disk (I suspected it would but was not sure).
I have not tested audio output from RGB21 connector yet, I suspect it is also stereo and is changed to mono by the switch but need to confirm it. I am not sure if my home build (by Omega) RGB21 to SCART cable is even wired for stereo 
.