One option is to get a subwoofer that is powered. That means that the subwoofer has an amplifier built-in. You would send the output of a pre-amp to the subwoofer, which splits the signal to the regular amp and to the built-in subwoofer amp. There are subwoofers that even allow you to use the speaker outputs to the subwoofer (different input jacks, of course) as an alternate way.

If you are using an A/V receiver, there may be a separate output for subwoofer, sometimes labeled LFE. Then you can get an un-powered subwoofer and add a amplifier, or just get a powered subwoofer and not need another amplifier.

I'm not familiar with this as some others in this forum may be, so please see what others suggest before doing something. Brands that come to mind are Sunfire, Hsu Research, Velodyne, to name a few.

Good luck and let us know what you learn and eventually do.

You'll have several options as noted above depending on the subwoofer you choose. You can split the pre amp output or run speaker level outputs to a sub which has provisions for such a connection. Find the sub you like and it will be pretty self explanatory how to make the connection.