Some interesting research I came across on the web:

The Upper Mid Range

Covering about one octave, this range of frequencies is responsible for the attack on percussive and rhythm instruments and the "projection" of mid range instruments. Equalization can be applied at any frequency in this range but still somewhat centers around 3 kHz.
On the foot drum, boosting 2.5 kHz or 4 kHz increases the attack. 2.5 kHz sounds more like a felt beater and 4 kHz sounds more like a hard-wood beater. These frequencies can also be used to increase the attack or "hit" sound on toms and snare drums.
Guitar lines often get more attack and distinction with equalization added at this range. A small boost (1-3 dB) for the vocal will increase projection. Adding too much energy, in this range, makes it hard to distinguish the syllables of the vocal and can cause listening fatigue. This range of frequencies is often reduced on background vocal to give them a more "airy" and "transparent" sound.

http://www.recordingeq.com/EQ/req0400/OctaveEQ.htm

Another factor is the harshness range around 2 to 4 KHz. A good mix will be very controlled in this range, again letting you turn up the volume without sounding too piercing. I listen to a lot of movie and concert DVDs, and the ones that sound best to me are never harsh in that range.

The Art of EQ

And a little off topic but was a bit of nostalgia for me:
http://www.bobhodas.com/tissue.html

When I was quite young I studied audio recording in college and very briefly spent time working in recording studios. The Yamaha NS-10M loudspeaker and the "tissue fix" was a long running joke in the industry at the time.

Maybe all I need to do is place a tissue over my Nrt tweeters!

DaveF would most likely not be amused!

-CB