WARNING!
The instruction as written may be hazardous to your equipment. I was taught that an amplifier should always have a load - this means that every channel of the amplifier should have a speaker or a resistor connected to it. To run an amp with the speaker outputs turned on, but with no load, might damage the amp. Can anyone else chime in with the electronics theory on this?
The correct way to do this, if you are using an amp with multichannel analog inputs, would be to disconnect the channel inputs to all but the center channel. The speakers would stay connected, but there would be no input signal other than the center. This would be safe for the amp.
The better option was the other suggestion - rip and boost, then probably duplicate the result to left and right mono.
I have always been a fan of listening to isolated channels of a multichannel mix. I also recommend checking the rears only of this mix. You can get a clearer idea of the surround intent by this method.