I got the idea to measure frequency response of various amps with free software and stuff around the house from this thread here:
Short summary, all you need is a good line-in, a 1/8 to 1/8 cable and the freeware program REW (Room Eq Wizard).
I used my Macbook Pro. I found I had to set the output to 44.1khz in Audio MIDI Setup in my MBP and set the soundcard setting to 44.1khz in REW.
Starting from top left, the orange is the HRT Music Streamer II (DAC only, no amp). Blue is the ODAC+O2 combo, Purple is the Headphone out of the MBP, and the turquoise is the HeadRoom Total BitHead DAC/AMP.
You can see the headphone out of the Macbook Pro is pretty good, with only a tiny treble boost. And the Total Bithead bass is rolled off.
The total Bithead also has a headphone processor switch, designed to give a less fatiguing, less in your head sound. I measured it with and without the switch below:
I could always tell the difference in bass when this is enabled, and now it's confirmed with this measurement. It's also pretty similar to stereophiles frequency measurement of both modes when they reviewed the BitHead (http://www.stereophile.com/content/headroom-total-bithead-headphone-amplifier-measurements).
When I used the O2 vs. the BitHead, I can tell the BitHead has less bass.
Obviously your line-in needs to be pretty good for this to work well but it looks like the line-in on my Macbook Pro does the job. If I get a hold of more amps I'll test them. If you guys try this out post your results in the thread.
Edited by moshen - 12/27/12 at 8:50pm