DT880 is analytical and detailed, very neutral (a slight emphasis on highs tonally), excellent clarity and airy feeling. HD595 is in warmer side. Its brighter than most sennheisers, but its still sennheiser. Balanced sound, bit tonally distant and soft, relaxed. Clarity and air suffered cuz of this.
Both are not known for their bass (though HD595 with its warmer sound might seem bit bassier at first). Their bass is fairly flat, not boomy nor hard hitting. Their weakest area if you want to have fun. However, if song really has deep and hard hitting bass, (as funny as it seems, many "bassheavy" music are recorded quite bass light...) they play it as such. They just not boost it. In technical POV, DT880 is miles ahead. More detailed and all. But it depends which you like sonically.