The PC market is dying, I think that buying a $3B company that will pay for itself in just a few short years is a better bet. Also, "shutting windows out" would be a bad move after they just brought Microsoft Office to the iPad.
HD600s are $380 on Amazon, and Grado SR125s are $150. I have owned both, and they are quite good - the HD580 was the top of the line model for a long time for Sennheiser, and there are some on Amazon for $190. I don't think I would buy the HD800s - I have dropped my HD600s probably 20 times when they snag on the arm of my chair, really expensive headphones are a bad idea I think.
It really is apples to oranges. Facebook paid a lot, but they paid in overpriced Facebook stock so maybe it balances out. Apple will be paying cash for a business that will probably pay for itself in just a few years via earnings.