I have a pair of Sennheiser HD600s that were sitting idle for a while, and wanted to put them to good use. As I contemplated buying or building a headphone amp, I decided to experiment with a very inexpensive solution, which happened to work with surprisingly good results.
I use a Placette Passive linestage as my preamp, which has 3 inputs and 2 pairs of outputs. I have a Wadia 861 going into one input (and a phono stage into the second input).
I went to Radio Shack and got a small project box, two gold RCA plugs, and a stereo 1/4 inch female headphone plug. I cannibalized some silver wire from a non-functioning Counterpoint amp, and wired the RCAs to the headphone plug, and put them all into the project box.
Then I hooked up my homemade box to the second output from Placette with short interconnects. With my main amp turned off, I plug in the headphones, and, Voila....music.
I am pleasantly surprised with the clean clear sound quality that this is producing.
I do not ever have the headphones plugged into my homemade box at the same time as the amp is on. But I do leave the box hooked into the Placette (w/o the headphones plugged in) at all times.