You may add any active subwoofer within your budget. I had this system in past and have connected Sony SA-WM500 . GN-88D have in built subwoofer in main front speakers. You can route LFE to main speakers until new subwoofer. Any thing upto 10 inch woofer within 25k may be good match. You may try forum classified section or olx for cheaper option. Your system may not do justice to anything more expensive than this.....