Sub-woofers have been around for a long time. The earliest ones I liked were built by Canton; a German builder who used a passive sub (non-amplified) with a pair of very small satellite speakers and gave a sound like you could not believe from this tiny package. Today’s sub-woofers are almost exclusively active (amplified). For a great inexpensive sub checkout the BIC V-80. It’s a great unit that has been on the market for more than 10 years. For something better, check out the same manufacturers as my recommended speaker builders.