Abstract: A musical instrument amplifier system (FIGS. 1 and 2) with a frequency selective crossover circuit (10) using passive band pass filters (36, 44) to separate the amplified signals from the power amplifier (14) of an amplifier system (12) into low frequency signals on an output connector (42) for a specially adapted bass audio speaker (38) while the higher frequency signals are applied to another speaker (46). The crossover circuit (10) is selectively connected with the normal speaker output (24) in lieu of the speaker (38) to add frequency crossover capability to any amplifier system. In another musical instrument amplifying system (88), the crossover circuit (FIG.