The PL50's response will also depend on the output impedance of the headphone amplifier. That's because the PL50's impedance has much variation across the audible spectrum:
I didn't measure much difference between coupled (worn) or in open air. DC resistance is around 27 ohms. The specs say 55 ohms at 1kHz, which is somewhat in agreement with the measurement. If you play the PL50 with a source with a low source impedance, like a Sansa Clip+ or an iPhone 4, you'll probably get the response shown in the graphs in the previous posts.
With a very low impedance sources, I hear too much energy at 2.7k and I have to EQ it down by a few dB. I also shelve the treble up because I feel that the presentation could be a little livelier. The spike at 8.5k is from ear canal resonance, and can vary from person to person. Some people lessen its effect with different tips. I'm comparing what I hear from the PL50 to a few pairs of speakers that measure flat at the listening position before I apply a house curve.
The PL50 might have been tuned for use with a source with somewhat higher output impedance, for example older portable players. This is what an iPad sends to the PL50 when there is added series resistance:
Except for the half-wave ear canal resonance, the response looks very much like my EQ correction curve.
The headphone amp on a typical notebook computer has high source impedance, so you probably won't hear the peak at 2.7k. If the output impedance is high enough, the PL50 will sound thin, lacking bass. If the notebook computer happens to be a newer MacBook Pro, which happens to have low (<1 ohm) output impedance, then adding resistance in series might make the PL50 sound better.
Some people like to use a 75 ohm resistor to change the sound of the ER4P to make it sound like an ER4S. (BTW, I'm not one of them.) Channel balance issues aside, you can do something similar with the PL50 with a passive in-line volume control like the ones made by Shure, Koss, or Radio Shack. Think of it as a variable P-to-S adapter for other kinds of single-driver balanced-armature IEMs. Just dial in the resistance until the PL50 sees the right amount.
Of course, you could just listen to the PL50 as it is, and just get used to the sound. I do that sometimes.
BTW, I also have impedance measurements for the A151. They're very similar. I also hear a response peak at 2.7k. Perhaps they really do share the same model driver. I can post the impedance measurement if anyone is interested.
Edited by yuriv - 7/15/11 at 3:26am