I agree with the title of this tread.
Before Xperia I had W595 and I was super-satisfied. Ever since my first mp3 player (SONY NW-E105) I stuck to SONY and didn't regret that decision since. Unfortunately, I was very disappointed after hearing sound on Xperia. Also, it is really strange that the native player doesn't include equalizer, a function so basic.
I compliment the SE's decision to finally switch to 3.5mm jack, and I really like new design of stock earphones (the track/volume control "button", it would be a pain in the a$$ to have to take the phone out of the pocket and unlock it every time I want to skip track) so that is why using some other phones is really not an option for me. I have to try some of the media players you suggested here to "fix" the sound with an equalizer.