I heard that the amp like O2 can improve the soundstage and sound signature so are there any good amps that improve both soundstage and sound signature? My max budget will be $300. Thanks.
Personally, I believe that headphones and source quality make the most difference in sound quality, followed by amps and (quite a bit further down the road) cables… I've bought a lot of amps, and, really, the main job of an amp is to get out of the way and, well, amplify the signal… there may be sound signature differences, some of them have hardware boosts to treble/bass/crossfeed/soundstage (and these do work, but they also alter the sound - if you like the alterations, fine, your ears should be the best judge), but, to my ears, amps usually do NOT completely change the fundamental characteristics of a headphone (unless, maybe, you go balanced - I have no experience with that, but Sorensiim reported very good results with his Noble K10, with improvements in soundstage too - on the other the K10 is already a TOTL CIEM, of course
).
If I were you, I would look into upgrading my headphones, first, either by going to ljokerl's large thread on IEMs (if you want to stick with IEMs) and selecting one particularly suited to your preferences and within your budget (and comparing it to what you already have), or changing altogether and getting some open headphones.
Best of luck in your search!
PS: if you really are set on getting an amp, you might want to try C&C BH2 - there's a large thread on that too tat you might want to have a look at, it's cheap (<$100), has a couple of hardware switches which enhance both bass and treble and soundstage, and it's really good for the money. There are a lot of better amps, but they also all cost a lot more
(and I actually found myself using it more often than my Arrow 4G or my Pico Power, with N6 IEMs with FW 1.2.6, while with 1.2.8 I don't use an amp - needless to say, YMMV, and I still think that you'd be better off by upgrading your 'phones
first)