I've been using an Asus Xonar Essence STX for years, recently swapped the op-amps for Burson Audio Vivid V6 and I've been very happy with my setup. However after I changed to planar headphones and started acquiring insanely high quality FLAC files of classical music, I've started to feel like my trusty soundcard isn't powerful enough, on some tracks I can crank the volume to 100 and it's a bit too much, but not painfully so. It's crazy how big of a variance there is on volume in different music.

So IME you only need an amplifier if you're listening to classical music.