1. DIY. With custom-wound output transformers with 4ohm, 20ohm and 300ohm taps. First stage: 6N1P-EV or Tungsram E80CC with EA tube voltage stabilizers. Last stage, military GU-50 or SRS-552N. Not sure about the max output power, but this is basically a speaker amp, so it's way more than enough to force any headphone to make you bleed to death through your ears.
2. DIY DAC/AMP - TDA1541A with NE5532FA for the output stage
Modded Raspberry Pi 2 with liner power supply (Linux ALSA and MPD)
Modded Qubietruck with liner power supply