Ever wonder what the other buttons do on this thing?

The 5.1 83dB SPL SMPTE X-Curve button does exactly that. When measured with a reference microphone at the listening position The Left front, Center front, and Right front speakers will each produce 83dB SPL individually, when each individual channel is fed a -20dBFS signal. The Left surround and Right surround channels will produce 80dB SPL, while the LFE channel will produce around 96dB SPL. Mostly the channels follow the SMPTE X-Curve frequency response (see the screen grab to the right.)

Flat turns

 
 

 
Mute mutes.