Someway, somehow, I don't know why, there's something really special in listening to SoI with a good headphone. Oh, yes, every music is better with great headphones, that's true, duh... but I think this particularly truer for SoI. Let me explain.
You know, SoI doesn't have any big thing that really stands out if you are casually listening to it in the car, like unbelievable climaxes, genius hooks, a spiritual atmosphere, crazy experimentation or lots of swagger. Far from it. SoI is just a straightforward record with a lot of thought to detail and craftsmanship. Just listen to Larry and Adam, they are beasts together in this album!
So, with headphones on, the songs get so tight! Every instrument clicks together so well. Everything is so well balanced together. It's amazing! Even if we can criticize the production for polishing the sound too much, making it too cutesy, on the other hand, they really did a great job at interweaving the instruments, like real artisans. There's a level of basic sophistication you don't usually see in other U2 albums (even if the others may be better with their respective strenghts). And that's where this album shines. That's why you need headphones, so you can listen to SoI for what it really is. You can hear each detail and appreciate the beauty of all their hardwork.
Just my 2 cents.