they will definitely sound better than the beats studio, the hd 650s arent too bad unamped, they still will not be as open or deliver the full sound of them with an amp, but they still will sound detailed and clean as any headphone in their price range.
beats studio = muddy bloaty bass thats not well defined, very recessed mids and basically no highs.
sennheiser hd 650s unamped = clean but anemic bass (you can hear it still but without the impact). recessed mids ( nowhere near as bad as the beats). clean highs (low volume will make them hard to hear), the soundstage will be small as well as having an overall veiled sound. however, they WILL sound ALOT better than the beats still. its not as bad unamped as i make it sound like. however, there is a definitely degredation in sound quality.
sennheiser hd 650 amped = beats killer. bass is tight and well definied, will have good impact on songs with emphasis on bass (most rap, techno, etc), smooth and warm mids that are well balanced with the rest of the sound spectrum, clean highs that are definitely more relaxed and never bright. a good overall headphone that doesn't fall behind on any one criteria.
note: portable amps by my experience are not powerful enough to power the hd 650s, you will hear in improvement in SQ but its slight and the volume will be barely noticeable. definitely consider purchasing a full size headphone amp.