You are correct. LPF = Low Pass Filter designed to send on ly the lower frequencies to say a subwoofer. HPF = High Pass Filter designed to send higher frequencies (block the bass) to the smaller remaining speakers. You would typically only use one set of filters either the ones n your deck or on your amp. Using both can create 'holes' in your frequency response making the system sound awful. Now to determine which one to use i'd suggest checking the manuals to determine which has the steeper crossover slope represented as ##dB/octave. 6,12,18 and 24 are the most common. The higher the number the steeper the slope. I also use the steepest slope available to the sub, but for the high pass i trust my ear, but usually I use 12dB/ocatve as a minimum. I would be hapy to help you choose if you can find the information on you equipment.