If you're looking for differences between this model and the MDR-V6, that model has a nickel-plated 3.5mm plug and 6.3mm adapter. Also, the MDR-V6's connector housing is matte silver, while the MDR-7506's is matte black.
I like that the MDR-7506's "L" and "R" markings are color-coded and easy to see in dim light. A no-frills black vinyl carrying bag is included. The headphone comes with a 90-day warranty.
Listen to the MDR-7506 and you'll know why it's remained in the Sony lineup for 22 years. Nothing about the sound is out-of-place: the bass-midrange-treble balance is accurate, and every music genre sounds great. It's no wonder so many professionals have relied on the MDR-7506 to record and mix music, radio, movies, and TV shows. Audiophiles on a tight budget will find a lot to love about this headphone.
The MDR-7506 sounded more open and less "canned" than the Noontec Zoro on-ear headphones. Switching between the two, the MDR-7506's stereo imaging was broader, less stuck inside my head, and the Sony was more comfortable and provided better isolation from external noise. The Zoro's treble detail is quite nice, but the MDR-7506 sounded more natural overall. The one area where the Zoro definitively trounced the MDR-7506 was volume capability; it could play a lot louder on my iPod Classic.
Impressed as I was with the MDR-7506, it made sense to next pit one of my favorite $200 headphones, the Audio Technica ATH M50, against the MDR-7506. The M50's sound is richer and weightier in tonal balance. The MDR-7506 is thinner and brighter, which I didn't like as much. The M50 is the better headphone, but it does cost nearly twice as much. (If you can afford it, it's worth stepping up.)
Comparing the MDR-7506 with the MDR-V6 was interesting; anyone who claims these two lookalikes sound the same should have their hearing tested. First, the MDR-V6 makes more and fuller bass while MDR-7506 is leaner -- vocals sound more immediate, and the treble range is accentuated. As noted, the MDR-V6 is comparatively laid-back and mellower, while the MDR-7506 crisper and livelier.
Aside from the fact that the MDR-7506 and the MDR-V6 have some drawbacks for mobile use -- namely the long, coiled cord and lack of an integrated microphone for cell-phone calls -- both headphones have aged extremely well and I'm glad Sony has let them be.
As noted, while the two models look nearly identical, they don't sound identical. I can't say they're day-and-night opposites -- the two headphones share a similar overall sound -- but the differences are significant, so it's just a matter of picking the one that matches your taste. I'd go for the MDR-V6 -- I highly recommend it -- but CNET Executive Editor David Carnoy, who edited this review, preferred the MDR-7506. And so did CNET editor Matthew Moskovciak.
In other words, these are two excellent headphones that offer distinct sonic differences. It's less about which one is "better" and more about which sound you prefer. But if you value sound quality and comfort, both of these should be at the top of your list when you're shopping for headphones under $100.