Re: factory recall of all S90es units...

just spoke with yamaha synth tech support:
(he seemed cautious about talking at first but then opened up)

Yamaha has confirmed that under unusual circumstances (very hot ambient, turning power off and on repeatedly), the headphone output (he didn't say the main outs) can produce a very loud, hearing-damaging sound. The problem is with a particular integrated circuit that has to be replaced to remedy the defect.

Yamaha expects to have all in-factory and dealer stock units fixed by around the end of this month and resume shipping (fixed) units to dealers early next month.

Recall of problem units already in customer hands will start soon and might even be on an advanced replacement basis (they ship you a new unit first then you return the old one!).