OK. VERY basic. Look up the crossover frequency from the woofer to tweeter in your current speakers. Next, do some research on a woofer that plays as low as you want, but also plays well beyond the crossover point to your tweeter. (Likely a six, maybe an eight inch driver.) Make sure you buy a woofer that comes with the T/S parameters so you don't have to get a woofer tester. Build the appropriate/ideal box for the woofer using the published T/S parameters and WinISD, a free download. Use the old crossover and listen to it, then come back here for fine tuning. It WILL need fine tuning. One cannot assume the current crossover is right for either the woofer or tweeter. Do NOT make your first project a three way until you go through the gyrations required to get a two way right. Ask me how I know.