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?
As far as I can tell, it's the same driver. I bought a replacement driver for my 555s and the packaging it came in said HD-555/595 driver.
More details on the modification process can be found here: http://mikebeauchamp.com/misc/sennheiser-hd-555-to-hd-595-mo...