[Dr. Strangelove]

I decided to put some new cable on my AT706 electret phones because I was bored... the fact that I had cut up the cable to direct drive them from my speaker amp provided a little motivation**. Anyway, after taking the cans apart, I noticed that the + lead goes to the metal grid behind the small driver membrane and that the - lead attaches to the membrane it self (the driver is sealed but I can see that the + wire goes towards the back of and vice versa for the -). Par for the course as far as construction goes... but I found that between the wire coming from the - lead and the cord exiting the can, is a small resister. 
 
I have no idea why this resister should be here and I'm curious if anyone with more knowledge of electrical can enlighten me. Mostly, I'm curious why it's there and if it might be beneficial to remove it.
 
Keep in mind, these cans are designed to be hooked up to a small transformer box, which is in turn hooked up to a speaker amplifier.
 
**Whoever thought that using a DIN plug was a good idea should really... not have thought that. It's so small that wiring it up makes for an incredibly tedious experience.

 

[JoetheArachnid]

If there's a resistor inline it's likely there to alter the sound in some manner. It really depends on the value of the resistor.
Most headphones will respond differently to different output resistances from amps - BA IEMs with crossovers tend to get fussy over any more than 1-2 ohms, whilst Beyers and some Sennheisers are designed for the theoretical standard 120 ohm output. A resistor inline with the driver is effectively adding to this output resistance, and is in the process lowering the power that reaches the drivers and changing the sound subtly. The power loss is inherent to more resistance, though this shouldn't bother you if you're driving the cans from speaker taps. A good rule of thumb is that low output resistance gives sharp highs and very tight lows, whilst higher resistances should tame the treble a little and give the bass some more kick. Often the reason a headphone 'synergises' well with an amp is because the output resistance suits that headphone especially well.
 
Of course, it could be rather different for electrets...
 
Broadly if the resistor looks like it's been there since manufacture, it's probably there for a good reason: either to alter the sound or to provide some function to the electrets. If you have a DMM to hand, check what value the resistor is. (I'm interested)