18 in. subs in a ported box should be good to around 15 Hz. Somewhere in the 16-63 Hz band you should hit 115 db. Do you have sub EQ and check the room's response like BH suggested?
Basshead81, I think I read you have some big Klipsch speakers?
Klipsch RF 7 based HT 7.4, Pioneer SC 35, Acurus 200 Five, Dayton 18 Ultimxa Dual Sub Cab(2), Dayton 18 Ultimax Large Vented Sub Cab (2), on Berhinger I Nuke DPS amps, Samsung BDP F 7500, Asus/My Book Live HPC 4 TB
Yaquin VK 2100 amp, McIntosh XR 5 speakers, Samsung BDP F 7500