[Spyro]

I have read many quotes proclaiming "most of the music is in the midrange so it is the most important frequecy."
 
If this is the case, why do V-shaped or U-shaped (for example W3 or TF10) sound so darn fun and engaging?

 

[Tronz]

I'm guessing it's because we're the most sensitive to the midrange frequency. As a result, boosting the bass and treble, brings it to a more easily perceived level balancing everything out. 

 

[joelongwood]

It's all about the Fletcher-Munson Curve. As volume decreases, our perception of bass and treble tones decreases and we hear mainly the midrange frequencies.  The music seems lifeless until we boost the bass and treble, hence the loudness button on many receivers and stereo amplifiers, which attempts to restore the missing bass and treble. Here's an explanation:
http://www.heilsound.com/pro/mic-primer/fletcher-munson.php

 

[molemonpledge]

Thanks for the link, very useful explanation.