I would personally stick with the original CX300's then use a bass emphasizing derivative. You don't want cans that are boomy, with muddy bass blowing out the mids and highs. I listen to a wide range of music, leaning heavily to classical, jazz, instrumental music. I prefer a detailed sound that gives you the "in the studio" effect. I want to be able to hear Santana's fingers on the strings of his guitar and Yo Yo Ma breathing as he pulls his bow back against the strings of his cello. You miss that unless you have balanced, neutral sounding speakers/phones. If you listening to the electronic sounds that are big on the top 40 charts with some rap, hip hop, you're "skull crusher" headphones are fine. IMO, the electronically generated music that is popular at the moment doesn't have enough nuance to warrant spending money on good headphones and you are better off getting the cans that are the most comfortable for you. If you're into bass heavy music, IEM's are probably the worst headphones you can use. It's impossible for those tiny little drivers to create the effect you want without distorting the rest of the sound field. You would be better off with a pair of on the ear opened cans, like the Sennheiser PX100's, which are far more comfortable than IEMs and fold up into a small pouch. They have really nice sound quality as well and the bass is surprising rich and deep, without booming and overpowering the rest of the sound spectrum. The downside on open cans is that you will have sound leakage, but if you want good natural bass and a broad sound spectrum, these will get the job done for under $50.00.
If you're using headphones for watching movie soundtracks, neutral headphones still make sense. You have a lot going on in a modern track that is typically 5.1 tracks which you are generally reducing to two LPCM high bitrate tracks for headphones. In this case, you want headphones that will give you a natural sound on dialogue with no hisses on the S's, crushed syllables of unintended variations in tone and pitch. You also want to be able to hear the "surround" effects that are built into the tracks... birds in the background, wind in the trees and, with action scenes, you want to get the full experience of the explosions. Headphones will never give you the sensation you get with a quality sub, but a good set of cans can still render reasonably tight bass.
Players: Ainol V8000HDW, Ainol V6000HDT
MID/Tablet: IPad 64gb Wifi
Computer: Dell Studio 15, HP Elitebook 2530p
Music: Sansa Fuse (8gb), Creative Zen (8gb) and Apple Ipod Classic (120gb)
IEM: Brainwavz M2, Brainwavz M3 (thnx Raz), Shure SE310, Etymotic ER-4
Cans: Grado SR225i, Sony MDR-V6, Beyerdynamic DT 770, Koss Portapro
Amp: FiiO E5, Boostaroo, Audiophile Fubar III