I just noticed this morning that my 
is showing a line of dirt/gunk beneath the triggers, similar to the stuff that accumulates on my aluminum Apple keyboards. Hand oil or dead skin or … I don’t know.
What are the best modern tips/tricks for keeping the silver boxes’ faceplates clean without damaging anything?
Sorry, I don’t have “new tips” to clean up my gear. I just use my grandmother trick : white vinegar on coton. Cheap, efficient, with no danger for the gear.
Be careful not put too much vinegar on Coton, just humidifier, no surplus or it could drop in the gear and there you’ll have problems…
Your gear will be as the first time you unbox it (I talk like in TV advertising).
Once you get it clean from the gunk, I suggest keeping a 2" wide paint brush on your music desk. It’s a great/quick way to keep all the dust off the gear. I do it every few days.