Do this and get extension down to the single digits. Easy.
104db at 20hz is not bad, add around 6db if you co-locate the other sub. If you need high output at 20hz then go with multiples. If that is too expensive then a big ported box tuned to 20hz will do.
Now compare TWO Titanics in a 7cu ft sealed enclosure with 1336w and ONE Titanic in a ported 8cu ft enclosure tuned to 20hz with 633w.
Sealed is yellow
Ported is gray