Keyboards that I've seen him play (2006) were mostly Korg Tritons (5 of them) plus a Yamaha board for piano sounds. In some other set-ups, I've also noticed an older Korg 01W and even a really old Emu Emulator.

'Yanni' is used when addressing the person; 'Yannis' is used when referring to the person.

So you might say, 'Hi, Yanni!' because you're talking to him. But when talking about him you would say, 'Hey, you'll never guess what Yannis did last night at the Vicar's house!'

The appearing and disappearing 's' is fairly common in Greek names, I believe.