@earmonger: thanks for the hint about the Audio Technica. It seems like an interesting option. After reading a few reviews the only problem / disadvantage I can come up with is its earcup and headband size: I guess I have a larger head size, and earcups and headband for the AT seem to be on the smaller size, compared to the other makes. Would this be a big deal you think?
I would prefer to try them out in a real shop, but after some online searching I didn't manage to find them in a not too far distanced shop. Would it be silly to just make a decision based on online reviews and buy it online without having tested and compared it yourself?
Maybe you can try to find a retailer that offers the possibility of return in a defined period of time. This way, if you don't like the product, you just send it back.
If you decide to go for ATH-R70x, you get the best performance form the beginning since it is a headphone that does not require a burn-in time to deliver the best quality.
Personally I'll choose Sennheiser, because of past experiences with the brand(I used to own 3 different models in the past) and I've got a new 660S for my office use not so long ago and I am extremely satisfied by all of them.