Upgrade value depends upon your system. In my modest secondary system (Yamaha AVR, NHT Super Zero Speakers, Pecan Pi Streamer) I recently replaced an old sub woofer with an SVS 3000 Micro sub. I followed SVS guidelines and changed the subwoofer crossover to 120 Hz while getting the new sub dialed in. The combination of the new sub and new crossover setting made a quantum improvement in sound for both two channel listening and for movies.
I think that the new sub woofer helped me to maximize the sound quality in this system.