I don't know if anyone has heard the KEF M500s here either. If these don't fit the bill for you and you like the sound of the Grado HP-2 or Kef speakers with a bit of isolation, the M500s should fit the bill. I've been thrilled with them.
I'm interested in hearing the HP-50s also. I'm not sure if I like the 1960's look though.
^ I came to the same conclusion. Originally, the mini was on my radar. My job gave me the option to order a retina mini or air. I ended up getting the air with a keyboard case. I'll probably be selling my Nexus 7 also. I felt that the N7 is a better fit one handed. The mini is too wide to be a one hander and too small to completely satisfy. I'm looking forward to getting the air.
edit: i'll probably keep the Nexus 7 for reading books.
The mini's PPI seems more appealing to me than the air's. As appealing as it is, I'll probably stick to my Nexus 7. Laptop+phone seem to cover everything but book reading for me. Aiming for the new MBP, however.