Hi Dusan,
The Tyros/Genos keyboards are all close to the standard "consumer-rate" Ohm output,which is 32 Ohm. I forgot where I was reading about it, but if my memory serves me correctly, the Genos and Tyros keys output maybe up to 40 Ohm - but I'd have to check again. So in other words, you are best off to use the standard 32 Ohm (or thereabouts) headsets.
My pro audio grade Beyerdynamic are rated 250 (250 - 600 is what you usually find in Studio and high-end cans) - and they sound OK on my Genos, but you can tell it's not what they are designed for, and I'd have to put them through a good headphone amp to get them to work 100% using the keyboard phones jack.
Hope that helps. Cheers, Roland