Here's one way to get you bass well-positioned in the centre: Use Mid-Side processing in an equalizer:
>>> http://i.imgur.com/Swr08pe.png
- set the Equalizer to Mid/Side processing (on the right-hand side),
- set the bands as shown, right-click the node in the graph or in the list,
- in the General section, select the Mid channel for Band 1 and the Side channel for Band 2
- tweak to taste