some pc speakers have a low pass and volume knob on the subwoofer to adjust how strongly and how often it fires. you can easily adjust sub presence in this manner. the z313 is basic and does not appear to have this option. looks like the z523, z623 have it and i know other models do. as for separate speakers and subwoofers (guessing you mean to buy an active (powered) subwoofer and a pair of active speakers) generally the sub will have low pass and volume knob on it but the computer will be limiting what gets sent to it as well based on the kind of content you have running.
without a low pass knob, not much you can do except using equalizer or a software low-pass filter to cut back on bass. since the satellites should not be outputting...