same sub, same amps, same xover, same tuning, same port area. one box is 6 cubic feet and another box is 8 cubic feet.   what will the audible difference be?  i know the smaller box will require more power to move the same amount but aside from power handling does the ability to reproduce certain frequencies become effected? 

In order to achieve the same tuning, your port will need to be slightly shorter if you increase the volume of the box. Power handling is reduced slightly due to mechanical limits. As for reproducing certain frequencies, a larger volume box will enlarge the peak that exists at the tuning frequency. It will thus be louder, but if the peak is very large then the other frequencies will be a bit quieter in comparison which can create an unnatural sounding frequency response with exaggerated loudness close to your box's tuning frequency (Fb). Can be good for SPL, but large peaks are not good for balanced sound or if you are going for SQ. To find the difference of how much larger the peak is, you would have to model it on WinISD or something similar to check if that is the response that you want. However WinISD does not account for cabin gain. 

i have spent some time in winisd today and i think i understand what i need to do next. im going to find a db meter and find my cabin resonance. from there im going to map out an EQ plan in winisd to flatten my response curve and then im going ot build a new box, buy a 1/3 octave EQ, and see where im at.