There is no functional difference. The parts that differ are the cord part number: the cord comes as a single unit in replacement catalog. The V6 cord has a nickel plated plug attached to it and the 7506 has a gold plated plug attached to it. The only other part number difference is the metal outer can covers. One has V6 engraved on it, the other has 7506 engraved on it. Other than that, all parts, including the drivers/transducers, are identical in cross reference between the two models.
The REAL differences are price and warranty. The MDR-V6 sells for $20-$30 cheaper. The MDR-V6 has a useless warranty based on what I have read. I have read accounts that Sony consumer division does not repair or replace the V6 when sent in for warranty services. Instead, they will send you a refurbished MDR-V600, based on complaints. It is a totally inferior device to the MDR-V6, but Sony considers it an 'equal' value item that is comparable. The 7506 has a useful warranty that will actually provide repair/service for the product. Sony professional is a completely different division than the regular consumer division.
-Chris