ohh... i think I understand you very well:
i own PX100 too, and i used them everywhere for everything, just love them, and i will too for a long time...
i upgraded to MS-1 and i think it was the best thing i did, they really sound amazing and in fact you perceive a great upgrading difference from PX100
although MS1 are not that portable (mainly compared to PX100) i love their sound so much that i use them as portable! (and i do travel a lot! - train, sub, ... ...) the only problem is that they are... ... open!
so, after digging some more, just ordered my real portable closed upgrade into AT - ES7!
take a look, i think you'll like... http://www.head-fi.org/forums/showthread.php?t=241060