Sub Woofer for Golden Ear Technolgy Triton 5

Depends on budget, and whether you want sealed or ported (expensive ported subs are very musical, even 18” ones). What are is your preamp/integrated/receiver?

Check out JL Audio. They should work fine with your speakers!
http://www.jlaudio.com/home-audio-powered-subwoofers

Sumiko S-9 or S-10. They are a Rel clone and ideal for music IMO. Integration is great and will make your main speaker sound bigger. I bought 2 S-10's from Paragon. They are on close out due to dropping the Sumiko name for Sonus Faber in the next ones made