You would hook up a "L-Pad" .

Or you could add a sub amp (these go on sale for $29 all the time):

http://www.parts-express.com/dayton-audio-sa25-25w-subwoofer-plate-amplifier--300-782

In a pinball machine, the watts delivered from the amp chip are so low that Speaker Efficiency is probably the most important thing to look at during replacement.

put a resistor say 20 watt, in line at the back box speakers, then you can turn up the volume, without the back box speakers blowing you out.

Putting L-pads and resistors in will work, but as vid mentions, valuable power from the amp will be wasted in the L-pad/resistor.

I suggested a much more efficient way here:
http://pinside.com/pinball/forum/topic/how-to-add-a-cheap-sub-under-40-that-sounds-awesome-to-your-wpc-game#post-1182133

I think it is THE most elegant way to do it (at least for 90s B/W machines), though my suggestion hardly seems to have gone viral... Be sure to read the post after that one where I make the mod even simpler.

The same technique could probably be done to any pin with a separate subwoofer amplifier. Send or post the schematic of the audio output signal chain and I can tell you what resistor to tweak.