Thanks, those tips should come in handly. The MMU2S seems quite technical with custom controller and all. Not necessarily a bad thing, it'll just take some time to get used to it. My current multicolor machines are lot simpler, more in line with the first MMU. I'll do a forum post about them and the MMU2 journey later. BTW, it seems the MMU2S is again out of stock in Prusa shop, seems there's shortage of parts again. Hoping my kit shipping isn't affected too much.

I ordered SKR Pro and TMC2209 drivers, figured it's time to upgrade. Maybe I'll swap it on the custom Prusa machine. Not sure I'll ever need 11 stepper drivers, but who knows where this multi-material thing is going . I have a cyclops-clone from a X5S-2E machine, it's not very good but does color mixing so MMU2S + cyclops would be interesting. Just thinking about the amount SW tuning is needed... . I do have 6-color machine with 6 extruders, but I'm only using one stepper driver to control them all. I use a custom relay setup to switch the active extruder when switching filaments.

RRF woes: setting the fan speeds lower than 500Hz has an interesting side-effect: setting fan speed to 100% doesn't work if it hasn't been set to something else before. I.e. right after starting the printer, issuing M106 S255 does nothing. But if the frequency is set to 500Hz, it starts to spin as its supposed.
On reprap forums one of the dev thinks there's a bug in the FW and hopefully there's a fix coming. Not a big problem because I need to swap the fan in any case as its axial and not powerfull enough.

Also having some quality issues with low layer heights (0.15). I use adaptive layer heights in KISS and that seems to cause inconsistent quality with RRF. It might be due to Pressure Advance, but haven't had time to test it properly. Also might be due to bad cooling so better fan duct is in order pronto. Might switch back to Marlin to get a reference Benchy done, luckily switchin back and forth is quite simple.