Hi all:
Thanks for your ideas and patience. I finally got my buddy's Fluke auto-ranging DMM and got to work on measuring the PA2V2, which I purchased from Electric Avenues about 15 months ago.
To first check the set-up, I measured the output of a Behringer UCA202 DAC/amp, which is known to have a 50 ohm output impedance. Using a 100 hz WAV test tone on my laptop as the test signal, and with a half-watt 100 ohm resistor as the load (98.6 ohms, to be precise), the Behringer measured at 50.4 ohms. Sweet.
Next, I sent the Behringer DAC's output to the the PA2V2 (operating on its charger), and with the 100-ohm resistor load, the output impedance of the PA2V2 was 2.52 ohms. Scaling down to a 11-ohm resistor, which being closer to the PA2V2's value likely made it a little more accurate, I got 3.29 ohms.
So, it looks like the PA2V2 has about 3.3 ohm output impedance. Headphones with the PA2V2 should therefore probably be at least 25 ohms or so to get good bass damping and a nice flat frequency response. With 16 ohm cans, this may cause the bass to be a bit flabby at higher volumes, and may lead to some minor frequency deviations if the cans themselves have a wide impedance range.