I have used the MDR-7506 phones for years for documentary location sound recording - they are the industry standard here in Australia. In fact a sound recordist I used for a doco series recently - Paul Brincat - uses them, and he has recorded many big budget features shot in Australia over the last few years (including the last two Star Wars, Mission Impossible II, Scooby Doo, The Great Raid, he was even nominated for an Oscar for The Thin Red Line). How he got to record some sound for a humble doco series is a long story - but he uses the 7506. They are bullet-proof.
The MDR-7506 for listening to music is another story - I recently purchased a pair of Beyer DT880's - and they are much more enjoyable to use with music - more open and natural sounding - I have many movie scores, and I've been listening to a lot of cruisy, chill music lately (Nitin Sawhney, Hotel Costes, 1 Giant Leap, Air, Thievery Corporation, Verve Remixed etc etc.
They all sound distinctly better through the Beyers.
The MDR-7506 are indestructible, and isolate location sound better than open or semi-open cans. They don't reproduce music as well as the Beyers, or Alessandros MS-2 or MS-Pros, or the higher-end Sennheisers.
My suggestion is this: keep the 7506, but purchase a range of other cans as you can afford it - if you're a composer and musician you will presumably want to know how your work sounds through a range of speakers.
I use ATC Active 10 monitors in the edit suite - they are awesome monitors - and have the most fantastic sound stage and instrument focus - love 'em.
Hope that helps