When do I need the optional BMW UG B100 XHP high-pass crossover?

The optionally available and individually packaged UG B100 XHP high-pass crossover is used for the midrange / tweeter when high-pass filtering is not available. The crossover filters out all low frequencies, so the midrange driver plays much more precise, higher levels can be driven. If you install a retrofit amplifier, an integrated high-pass filter is usually on board as equipment, so the optional crossover is not necessary, the high-pass filtering takes place directly in the amplifier.