Wrong. You definitely don't need an amp for those and in some cases an amp could even reduce the sound quality of HD 598's cause they are such high sensitivity headphones. Also, the HD 598 have an impedance of 50 ohms, so for accuracy and fidelity you'd only need to make sure the source's output impedance is pretty low (< ~12 ohm). And even if your source had a high output impedance (most soundcards and circuits have like 15-35 ohm output impedance), that is not a very big deal and some people would even argue they prefer the more boasted bass that the high output impedance of the source can cause.
It is headphones like Philips X1/2 that have a very warm sound signature with a boomy bass and low impedance, that truely benefit of a source that has a fitting output impedance.