Installed my HiFiBerry DAC in my Bottlehead Crack Tube Headphone Amplifier today. Mounted on a Raspberry Pi A+ with DAC+.
Routed the 3.5mm cable to dual RCA, fits perfectly under the feet, also the Pi USB power cable (not depicted).
Listening now, sounds amazing and I have a self contained, streaming tube headphone amp which can be controlled via iTunes as an Airplay device using Shairport (Linux Airplay Emulator).
HiFiBerry DAC+ in Tube Headphone Amp