Please don't actually buy HD 595s. The audio driver is exactly the same as the one contained within the HD 555s, the only difference besides the price tag and model label is that they just stick an (easily removable) sound dampener inside the 555s. Buy some HD 555s and mod them, you'll save yourself some money on a pair of fantastic cans.
Do they test the drivers and put the better ones in the more expensive headphones? Or do they just make two different headphones using the exact same driver?
More details on the modification process can be found here: http://mikebeauchamp.com/misc/sennheiser-hd-555-to-hd-595-mo...