Tony, play them at normal levels for about 2 weeks. And yes, be careful not to blast them...the diaphragms are rather delicate and don't enjoy loud bursts of sound. On another note, you are using the foams with them right? Just about all earbud reviews are based on having the foams on, because the foams increase the bass and warm up the midrange, so that the earphones don't sound so thin and tiny.
The MX-500s and 888s are very similar in sound. The 888s are still a bit better and give a bit more detail, so essentially if one has the money and wants to shoot for the best, the 888s are the way to go. I think they fit much better than the MX500s as well because of how thin their pipe duct is...the MX500s have that thick, plastic piece, while the 888s have an entirely bendable rubber piece. Of course the thiness of the 888's duct is part of its downfall too, since that means it's connected to the driver housing by only a smideon of rubber, and all it takes is a good chuck around a bag to seperate the pipe duct from the driver housing. Which is why, as I said, I use MX500s nowdays, regardless of price or sound.