Keep in mind I am using a sony discman model number D-E456CK from late 1999 as source and no amp to speak of except the one in the discman itself.  For classic rock like Led Zeppelin and Pink Floyd no problem what so ever but for aggressive stuff like Opeth's still life album or an Iron Maiden or even Judas Priest album you may want to look at a different can anyway.
 
But also keep in mind I don't own an external amp currently or even have one at my whim at the moment.  I personally think even a decent enough cheap amp {around $200} will do wonders for the sound by opening up the low bass and evening out the mids.  I do not personally believe that an external amp will do nothing for the sound however that may just be my opinion I could very well be wrong.   But I don't think I am mistaken in any way.