The Grados are quite sensitive and so will work with an mp3 player, but using a portable amp as well will bring out more of those headphones potential. If you are planning on using them be aware they leak sound, dont isolate you from outside noise and are not ideal for commuting or working in an office near others.
I would be using them whilst travelling, how big are protable amps seeing as saving space in my backpack is important and can you recommend any? I've heard of them leaking noise, this is not to much of a worry as I would use them mostly whilst alone. Thankyou for your help.
The FiiO E5 is tiny and available from Amazon and on ebay for £20 or so. It needs to be recharged via a mini USB, so it is designed to work with a PC. The even smaller E3 works with an AAA battery and cost under £10. Which one you choose will depend on whether you can get the batteries or have access to recharging.
I have taken my Grados away with me and I took my FiiO and ipod classic. To protect them I cut out spaces into the foam in the box the Grados came in.
An alternative to the Grados would be the Goldring NS1000. They have a similar sound to the Grados, cost less and come with a brilliant case for travelling. They can be used with or without Noise Reduction and both isolate you from outside noise very well. The Goldrings are very insensitive and really need an amplifier to drive them properly.