Please do not buy the HD650 with the intention of using them unamped. If you wish to invest later in an amp, go ahead and go for it as they still sound pleasant without an amp. However, you would be doing yourself and the headphones a major disservice if you were to buy them and never amp them. You would only set yourself up for disappointment. To be blunt, you would get very sub-par performance in a headphone capable of world-class performance. You would be much better off with something like the HD598 in that case.
That being said, my recommendation amped would easily be the HD650 as it is a beautiful sounding and immersive phone when driven properly. It is also one of the only reasonably-priced open headphones to have considerable bass presence and impact. Unamped, there are many more choices and I am not knowledgeable enough to recommend any, although you may want to take a look at the Denons that MalveauX recommended.
