S950 turned out to be rather problematic with its buttons. Had the power button fixed by a service, too, but other buttons become less responsive over time as well - although I had the contacts cleaned in the service, it seems that the problem reappears rather soon.

The off-function didn't bug me that much, I got used to disconnect the cable from the AC adaptor (although doing that a lot makes the cable rather loose).

The service center didn't charge me for fixing the button, but they charge a lot just for the work they do, no matter how simple or difficult it is.

In any case, fixing these small problems (like buttons, contact strips, pitch bend wheels) can be costy and troublesome, but it's much cheaper than buying a new keyboard, even like PSR-S670, in some way we are hostages of our boards.