[fnordfnordfnord]

>Of course, Dell could include analogue filtering circuitry to filter out the excess power in inaudible frequencies if they wanted to.

It likely isn't necessary as it is probably built into the output amplifier. I have no clue what audio amplifier dell used but here is an example of a typical headphone amplifier IC http://www.ti.com/lit/ds/symlink/lm4881.pdf

If you look on page 9 of the datasheet you'll see its frequency response. The attenuation is > -20dB past 15kHz at best. You aren't going to get much of a square wave above 1.5 kHz, and probably not below if you look at the response of the speaker.

[caf]

The lack of response of the speaker is the problem. If the power at those frequencies isn't being efficiently converted into sound, it's being converted into heat instead.

[simpleigh]

But those frequencies don't get to the speaker 'cause the amplifier doesn't send them...