Thanks so much for your kind words! I’ll do my best to answer your questions. The V6/7506 is my personal favorite overall of those three models, for reasons I’ll explain below. You can’t go wrong with any of them however, depending on what you’re looking for in a headphone.
Of the three models you mentioned, I prefer the sound of the V6. It’s more neutral and accurate to the original recording. I might be a little biased to this type of audio because of my production background. Are you looking for a crisp, high-frequency-focused rendering of your music that brings out all the detail (and flaws?) Then go V6.
Are you looking for a fun, relaxing listen that you can be confident won’t fatigue you even after hours? Then the 100AAP is the better choice.
Do you like bass? The MDR-1A would serve you the best of the three. It’s boomy and thumpy, like a cinema speaker setup.
The 1A is very comfy, and the 100AAP is almost as nice. They both have plush earpads and headbands, and they’re easy to get a good fit with: just adjust to the right size and pop them on your head.
The V6 is quite comfy too, but a little more utilitarian and plain. Its pads are smaller and less pillow-like. However, it’s super light and quite adjustable so you can find a great fit with a little more work. I have no complaints about comfort with any of the three, and can wear them all for hours without pain. But the 1A has a little something extra in this department.
This is where the V6/7506 really shines. It’s significantly cheaper than the other models, and has a sound and design that have stood the test of time. You can get the V6 for ~$80 to ~$100 US dollars, which is less than half the cost of the 100AAP. Now, the V6 is not stylish at all. It’s clearly a production tool that was designed in the 80’s. But if you want the most sound quality for the lowest price, it’s an exceptional bargain.
Do you want something stylish? The MDR-1o0AAP is a great choice. It’s perhaps the most subtle and sleek headphone Sony has ever released. It is high-res compatible, and only slightly less detailed sounding than the V6. In its price category it stomps all over things like the Beats Solo 2 for comfort and sound.
Do you want luxurious comfort, style, and oodles of creamy bass? Got lots of money to burn? The 1A is a great choice. I don’t think it’s necessary to spend that much though, the other two are better values.
I hope that helped. Sony’s whole headphone line is pretty solid. They balance sound quality, comfort, and value quite well, and so far I’ve never met a Sony headphone I hated.