Zapco Studio X Series - Manual (Page 11)

The ST-1000XM and ST-1500XM Input side
The larger Studio X bass amp s have all the features of the ST-500XM but
they also add more bass control features
1 2 See ST-500XM
1. The ST-1000XM and 1500XM offer a variable bass boost. This frequency
control varies the boost frequency from 30Hz to 80Hz
2. The Boost control allows a bass boost of 0dB to 12dB.
ST-500XM output side
1 2 3 4