I've never thought that I'm living in a so sound-technically advanced country. Because all handy recorders and all digital pianos that I've been interested in and researched so far were Japanese...
Yes, but when it comes to microphones and headphones...
Sennheiser are very German all right. The same stands for Beyer too. Prices in Greece are almost the same as in Germany. But I think in German e-shops you could really pick a bargain on these. There are "Compare prices" services available on the Internet. I bought mine from a German online shop this way at very low prices indeed.
A "closed" headphone is one that offers increased isolation from external sounds. It creates a space between your ears and its small speakers and it is designed so that external noises are cut out, sometimes very effectively. This way you can listen without interference in the subway, in a noisy environment etc. These could be used for example by a DJ in a club, or by a drummer in a live situation. Also in a studio, where the singer wants to hear the band in order to sing but the sound engineer needs only the singer's voice on the microphone, nothing else.
An "open" type is one that does not isolate your ears from the environment, so when it plays, it can be heard by others nearby and you can hear the others also. This type offers (in some cases) tremendous sound quality at a very affordable type.
A closed headphone is almost always inferior in terms of sound quality to an open type. If you don't play the piano in a very noisy environment, then go for an open type. The DT 770 Pro is a great set and you can also check the Sennheiser HD 515 or the very popular 555 at the same price tag.