First off, what kind of music are you looking to use?
Second, since I commute via
NY subway, sound isolation was a high priority of mine - and therefore, I focused on earplug-style earbuds.
In terms of jazz music, encoded into OGGs varying between 150 and 256 kbps played from GS Player, my X50v greatly improves upon the gain of my X30h (and certainly against my X5 Advanced, whose
jack disintegrated from wear). Using
Etymotic Research ER-6s, I no longer need a headphone amp to achieve suitable volumes without distortion, and in fact keep it a notch below its maximum.
And, just like my X30h, the sound quality playing
Oliver Nelson using a
CMoy amp is, in my unesteemed opinion, astoundingly rich and detailed in all regions (though especially in the mids to treble area). Although, because the gain is satisfactory on my X50v, and my CMoy is too bulky to use in the slimmer pockets of my Spring->Summer outerwear, I've been leaving home without it more often. Sound feels quite a bit harsher without it, but it's still satisfying.
However, again, much depends on the music played. By comparison, my
Sony EX71SLs, which do not benefit as much from an amplifier as the ER-6s, sound like AM radio playing jazz - but their artificially expanded bass and mid makes
Orbital plenty fun to listen to (so I have unique tastes, okay?? ;) ). The fact that they are typically less than half the price of the ER-6s, and provide pretty darn good sound isolation, and sound great with basically all genres of music except those which command high detail in acoustic instruments, make then an easy recommendation.
The X50v is about as good a portable source as I've heard, so the sound is as good as you're willing to invest in good headphones. And if you have the money, you won't be disappointed with a pair of Etymotics. But the EX71SL's, which I got for
$40 (saved on shipping since the store's in Manhattan), will please most of the time.