how can i get good bass form my xr3i cab because some people have said that you cant hear the bass from the boot of a cab properly. any ideas???

Morning everyone, hope the weather is better where you all are than here!   Have made a start on the stereo install in the cabby and was wondering if people had any suggestions. Have fitted my Kenwood dvd/screen unit in the standard place on the dash. Am now left with the lower part of the dash by the bas of the gearstick. I have a din sized hole here ( thinking gauges maybe? ) and then a half size which im guessing used to be the coin tray before the previous owner cut it out. Thinking of put

any one know what size the orignal speakers are?     and what it the max size i can replace em with... just simply by putting new ones straight in ?       please help ... my pink cab doesnt sound too good at the sec thanks