The 7506 is the Sony Pro Audio version of the v6. It's the best headphone Sony makes short of the CD1700 which costs quite a bit more. However, I would buy a V6 instead since it can be more easily found and found cheaper. Go to shopping.yahoo.com and enter Sony V6. You'll find them as low as $55 typically. Enter the same search with Sony 7506 and your lowest price will be about $90.
As far as I know, the 7509 is based on the V600 design. Many people prefer the V600 to the V6 but none of them dare speak in this forum. The V600 sounds remakedly like a normal headphone with the megabass left on--boomy, muddy, heavy bass that distorts or covers a good portion of the rest of the sound. I usually see the 7509 for about $200 and can't imagine anyone paying that considering the other options. Buy you know, some people love the Sony name and this is the most expensive Sony you can buy in normal consumer retail outlets and the 7509 version is typically the most expensive Sony you can find in pro audio shops.
I don't remember the 7505 specifically but can only offer you this warning--if it's not the V6/7506, I would keep your money very far away from them. The only consideration you MIGHT have is if you want in-the-ear design, the 888 is thought of highly amongst some Head-Fiers, I've not heard them.
So in order, my first suggestion is to look at headphones other than Sony. If I can't convince you of that then definitely get the V6/7506.
Kelly