Hello! Thank you for reading and for the question!
The 1A is much more comfy than either the 7506 or V6, and also much more focused on powerful, pleasant bass notes. I actually wrote a review of the DAC version over here.
The other two headphones are mostly centered around production tasks, though I also find them pleasant for listening. Some people don’t agree with me, because their treble can be a little harsh over longer sessions. It’s a matter of personal taste.
The 1A has a sound that should be pleasant for just about anyone, sort of like a Bose headphone. It’s not the most detailed thing in the entire world, but it’s very fun to listen to.
So, do you want the absolute pinnacle of comfort and love some good bass? Get the 1A. It’s usually more expensive, but its comfort and sound are designed for a higher-paying audience.
Do you want to hear all of the technical details and flaws in your music in a reasonably-priced and comfy package? Then the V6/7506 is better. There’s essentially no difference between them outside of the stickers, in spite of what some reviews have said. I literally cannot hear what they are talking about, myself. So feel free to buy whichever one is cheapest if you’re going this route.
I can’t pick your preferences for you, so from here you’re on your own, but I think they’re both good choices depending on your tastes. Have fun! And don’t forget to buy from a place with a return policy, just in case!