[dungeon]

 
I've owned my pair of MDR-7506s for over a decade and they are great headphones.   Ultra durable, comfortable, and they sound great ... with some reservation (read on).   For comparison, I also own a pair of SR80s and portapros and love them, but I can't commute with open headphones.
 
The problem is that recently, I do most of my listening through an iPhone 3G or a 2nd Gen iPod touch.   My SR80s and Portapros sound great though these players, but the Sony's are lacking.   I know that the MDR7506 are capable of more because I just did a side by side comparison with the same track coming out of a Tascam FW-1884 mixer and the details are there.  Played through the iPod and I don't hear as much detail.  It just sounds dead in comparison.
 
So, what do you guys think?   Try and make it work with an amp, or jump ship and try out a whole new set of headphones?
 
I've heard good things about the ATH-M50.  More bass would be nice, and if they have the same level of detail with good isolation, I'm game.   I am not convinced that I'd get much use out of an amp with my other headphones.

 

[Digital-Pride]

You've had your trusty MDR-7506s for quite some time now, so maybe it's time to upgrade, at least try something new.
 
Here are my recommendation for ya:
 
Sony MDR-7509HD, MDR-900HD
 
Shure SRH440, SRH840
 
Ultrasone HFi 580, HFi 780
 
and of course the Audio-Technic ATH-M50
 
Now all of these 'phones are excellent options, all have the foldable design, and are driven well by portable sources like your iphone.  They may not have the forward and bright presentation of your Grados, but are capable performers nonetheless.  However, as you move up the ladder in terms of headphone quality I would strongly suggest investing in a good quality amp in the future to maximize the quality of music from your headpones.

 

[Ouspensky]

I have the 7506 (beyer pad mod/more bass) and love it. If you liked it so much for a long time, you could just get a cmoy from ebay for less than 50 bucks, it will drive your sony's great and you will be able to commute with great sound.
 
that being said, digital pride's suggestions are good, but they would also require a little amp to shine their best. I say, get the cmoy, keep enjoying the 7506 while you do further research and go for a big big upgrade in the future, play it smart and you will save bucks...

 

 

[DjAmTraX]

I don't think the MDR7506 will sound that much better with an amp.  It's pretty good without one.