I had the Sennheiser CX300 from 2008 to 2012, they were my first good quality IEMs and I really enjoyed them. At first I remember the bass being pretty muddy and I had to turn my iPod/iPhone EQ to "bass reducer" or whatever it's called, but after burn-in they became much better and I turned off the EQ for the rest of the time I owned them. They were good sounding IEMs to me.
A few months ago, the right side gave out so I started shopping around and ended up with the Klipsch S4i II. I realized I missed the J-cord and was annoyed by the iPhone controls on the cable, so I shopped around for new ones and ended up with the Sony MDR-EX310LP.
I like the EX310 better than both right out of the box, no burn-in. Much clearer bass than the CX300, deeper/punchier bass than both the CX300 and the S4i II, much clearer mids than the CX300, and softer but better highs than the S4i II. The CX300 sucked for highs so there's no comparison there.
I'm really really happy with the EX310, can't imagine what they'll sound like with more than 2 hours of burn-in. I've only had them since Friday and only used them for the second time today.