Hyde@ShawnP.Watson posted earlier saying that he prefers the K553's to the L1 by a large margin, though many in the L1 discussion are naturally posting the other way. Here's a couple reviews from a widely respected audiophile (who loves both sets, but for different reasons):
L1: http://www.innerfidelity.com/content/handsome-philips-fidelio-l1
K550 (most are reporting that the 553 is a slightly better 550): http://www.innerfidelity.com/content/wonderfully-competent-akg-k550-sealed-headphone
Interesting note at the end of the L1 review: "For better sound at this price/type, check out the Denon AH-D2000 and AKG K550, but these cans won't be as versatile as the Philips L1."
Also on the sales, you're probably not counting the previous 1800 units of the K553's that sold before the drop re-opened =P
MeteoImpactYes, the L1's do nothing wrong. They are truly versatile and wonderful all around-er. The build quality is phenomenal and they're very comfortable.
But the M50x bested them. And the K533 bested the M50x. Therefore the K553 beats the L1. Look at the current drop....look how few L1's are being sold. Now look at the K533: 1800 sold on the first drop and 1172 on this recent drop. Go figure.