I own the MDR-7506's and a pair of Senn. HD6xx. I agree, the MDR-7506's are very good, and they were my only 'phones for years. I also cannot find a measurable difference between the two pairs.
That said, after using both for months and months, I can tell you that I prefer the Senn.'s. The MDR's are good, and in good per dollar they certainly win. But, I find the Senn.'s more comfortable over long periods, and less fatiguing to my ears.
This is, I believe, because the open 'phones let my ears breath, and because the sound on the Senn.'s is clearer/crisper. I wind up turning the volume up more with the MDR's, subconsciously seeking the clarity of the Senn.'s, which is more fatiguing for my eardrums.
That said, after using both for months and months, I can tell you that I prefer the Senn.'s. The MDR's are good, and in good per dollar they certainly win. But, I find the Senn.'s more comfortable over long periods, and less fatiguing to my ears.
This is, I believe, because the open 'phones let my ears breath, and because the sound on the Senn.'s is clearer/crisper. I wind up turning the volume up more with the MDR's, subconsciously seeking the clarity of the Senn.'s, which is more fatiguing for my eardrums.