for people that build boxes

set to low pass on a mono amp.  my question is setting the low pass filter to say 125hz means that it will play tones UNDER that frequency? (is the assumption that high pass is disabled since its set to low pass)

Also how do the high pass and low pass filters affect the speaker when set to Full range?

That really has nothing to do with how the box was built but you are right on.

If your sub crossover is set to 125hz it will only send frequencies below that to the sub.  In full range mode it sends all signals to the sub.  There should also be a crossover slope that will tell you how steeply the crossover cuts off frequencies outside of the range (it isn't an immeidate cut in most cases, more of a fade)