will a Sennheiser HD 650 work properly on the build-in plug of an Accuphase E211? Do I need a extra headphone-amp? Is it worth to use an headphone-amp instead (lets say max. 250$)?
It will work great out of the E211. Whether you like the sound or not is the question. But it should probably be fine. The HD650 is not difficult to drive. It's higher impedance so it matches well with the output impedance of something like the E211 (as most receivers are never 0ohm output impedance). I wouldn't bother getting a headphone amp when you already have this unless you find it to not sound well matched together (such as hiss, noise floor, weird effects, etc). If it sounds fine, then save your money and buy some more music.