From the xmms user manual:

3.3. Equalizer
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To access the Equalizer, select the button labeled "EQ" on the right side of the XMMS console.

That will bring up the Equalizer window. It looks like an equalizer on stereo and behaves like one as well. Press the button labeled ON to enable the use of the equalizer, once you turned it on you use it as a normal equalizer.
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On the left side you will see a slider labeled "Preamp". That will allow you to boost the volume.