>>596 ここのページ（Input Capacitors for Headphone Amps、ttp://tangentsoft.net/audio/input-cap.html） の結論のところに、こういう一文があります。
If you don't put an input cap in your amp, you avoid these issues entirely. There's no phase shift, no bass roll off, and no loss of sound clarity. Just realize that you risk damaging your headphones if you do this. A source that has a low DC offset today may develop a problem tomorrow.