In system 11, the cabinet and backbox speakers are connected in parallel. The small backbox speakers have very limited frequency range at lower end, while the cabinet speaker could reproduce much better bass. But if you crank up the volume, the backbox speakers usually sound too loud.
Making the cabinet speaker response only to bass frequencies, and reducing power to backbox speakers results in nice deep sound. This is accomplished by inserting a low-pass filter in one of the cabinet speaker leads - I used an old score unit coil, probably a 23-800 or similar would also work. For the backbox speakers, I inserted a 27 ohm resistor to speaker feed. When doing this, you have to check out which of the yellow/black wires on the left speaker goes to sound board and which goes to the right backbox speaker. Keep the speakers connected together and put the resistor to the wire coming from the soundboard.
Of course you may experiment with different low pass filter coil and resistor values. Cabinet speaker low pass filter can be tuned by inserting a discarded coil plunger partially inside the coil. The more it is in, the lower is the filter frequency.