I think Sony MDR-EX1000 is one of the biggest soundstage IEMS I have heard but my experience is limited , with the appropiate music the soundstage is considerably bigger than the ATH-M50
The sound of the EX1000 is kind of bright with very well controlled bass
In words of |joker|
I don't think I've heard anything that sounds exactly like the EX1000 with more bass. The EX1000 has a very wide and open sound to it that most other in-ears can't really match. Maybe the FX700 if you are willing to give up a little of the soundstage and clarity. You could go the cheap way and try an EX600 or MDR-7550 equalized to your preferences.