I will be getting a pair of Etymotic ER-4S ear canal phones and am looking to get an amp for around $150.00 or less (but may be willing to go higher). I would like the amp to be portable. I will be using it with a Sony Discman D-E905 and will be using it almost exclusively for classical...

I listen to classical music (Beethoven, Mozart, some with strong vocals). My source is an unamped Sony Discman D-E905. Right now I have a pair of AT A900's which sound pretty good but I am always willing to change if I can get something better (and I plan on staying unamped for now). I am...