The 6400/6500 have a very deep, resounding startup sound (which I happen to love) because the internal speaker assembly (the big box with a speaker at one end that you found) was designed to function as a subwoofer if external speakers are hooked up. The little knob next to the monitor port controls the amount of bass when you hook external speakers to it.

It's for this reason that my 6500 is my stereo. That subwoofer can be pretty kick-ass if you adjust it properly