There are two possibilities I can think of here.
1. Take your .wav and load it into a wave editor program like wavosaur (if on PC):
http://www.wavosaur.com/download.phpGo to the `process` menu,
Choose: Normalize > 0dB
Then save as Wav in the file menu.
or Audacity (mac + PC) [able to normalize too]
http://audacity.sourceforge.net/download/OR
2. Add a limiter VST to the master channel in Notions mixer before you export. You can increase the volume with that processor, which is designed for this purpose while preventing clipping.
This will however have ane effect on your audio dynamic range, but can be done to varying degrees on different styles of music to taste/ convention. Worth experimenting with. Here are a couple of free ones you can download to try:
http://www.gvst.co.uk/gmax.htm(classic master limiter located under free plugins):
http://www.kjaerhusaudio.com/download.php HTH