So I got T51i (couldn't find the p) and Kef M500 in yesterday. Did an A/B/C with the HP50 last night and had the wife serve as a 2nd opinion. Our mini-shootout was: HP50 > T51i > Kef M500.
HP50 is just more spacious - everything has room, and that is especially helpful in dense mixes. But the T51i is a surprisingly good headphone. I vastly prefer it to the HD 25 Aluminum that came and went. Much more balance in the low end, and no shrill highs. With the Kef - I really want to like it as it is beautifully made. And in fact for poorly recorded music it actually may have an advantage. It is very laid back in the treble but does have decent mid presentation. So actually rock electric guitar sounds really good through the M500. But there is no "air" to be found in it. Not sure if it is going to stay around. Not a bad sounding set of cans, but just not as involving or detailed as the T51i.