From the Everglide’s website we can glean a little in the way of specifications:

Technical Specifications

Transducer	Dynamic
Nominal Impedance	16 Ohm
Max. Sound Pressure Level (SPL)	102 dB
Max. Power Rating	100 mW
Weight w/o Cable	350 g
Cable Length	3.1 m
Frequency Response	20 hz ~ 20 khz

Everglide doesn't provide much in the way of specifications, here.  The 16 Ohm impedance means that these cans are a fairly easy load to drive, and will probably work with any sound card or portable player out there.  Usually the SPL is measured in dB/V, so I don't really know what Everglide means here with Max. SPL.

For the frequency response, I used George Taylor’s nifty ASIO signal generator (thanks, George!) and my ears to roughly guess the frequency response of the s-500 based on the volume of the notes being played.  I used an X-Fi for playback (volume set to 50%), because our usual X-Meridian's ASIO implementation is only 24-bit, not the 16 or 32 bit that George's invention likes.  Edit: George emailed with an update: his ASIO signal generator now supports 24-bit.  More info here.

For reference, a piano has a range between 27.5Hz (A0) to 4186Hz (C8).  Middle C is called C4.

Now then, using my ears and the signal generator, I found the s-500’s bass very strong down to about 40Hz (E1), and dropped off slowly till about 25Hz (G#0) where it was audible, but quiet.  Anything below 25Hz was non-existent, but sounds that low tend to be more felt than heard anyway.  This is excellent bass performance, by the way.

From about 40Hz (E1) on up to about 110Hz (A2), the s-500 was good and strong.  From 110Hz to about 1050Hz (C6, roughly, two octaves above Middle C) it stayed pretty flat.  You do want these frequencies to be flat since this is where most of spoken voice is at, as well as gun, and uh, chainsaw sounds.  Above that, between 1100Hz-1300Hz (C#6-F6) there is a noticeable trough where the sound is slightly quieter.

There’s another trough between 2400Hz-3500Hz (D#7-A7) and then it’s is up, up, and away to a big spike centered around 4200Hz (C8, just above the last note on the piano).  It seems to me that this is the main resonant node for the earcups.  You can still hear this note when you listen to all the other notes, especially the bass, and it is rather dissonant.

The s-500 stays fairly flat again between 4400Hz to 7500Hz (C#8-A#8), and falls off by 7900Hz (B8).  This is where a lot of cymbals are tuned to.

For upper treble, I switched to a test CD (Stereophile Test Disc #2) where I traced the response up to about 14KHz, where the s-500 drops off rapidly to an inaudible 16KHz (I can’t hear much above 16KHz anyway, so if they do go higher, I’ll never know it).

I will say that the s-500 are slightly better than your average pair of sealed headphones, but still can’t manage to escape from the funky plastic sound of the earcups.  Other than that, the frequency-response is quite similar to my reference pair of Grado SR-125’s, but the s-500 have much deeper bass.  The Grado’s don’t have the resonant sealed earcups, of course, so sound much more accurate.  I would love to review a non-sealed version of the s-500, if Everglide made it.  That would be sweet.