notnyt knows what he's talking about, he's been doing this quite a while. He's trying to help you out whether you want to think so or not.
If I recall correctly, the larger drivers don't work well in tapped horns because they unload earlier than the smaller drivers and then you lose their advantage.
The Danley DTS-10 has dual 12's that reach down to 12hz or so before they roll off. AKAIK that's about as good as it gets in a tapped horn.
Television: Mitsubishi WD65737 DLP
Processor: Emotiva UMC-200
Amps: Carver AV 806x/Behringer EP4000
Mains: DCM TimeFrame 600 Center: AT 453C
Surrounds: AT 251.1 Sub: Danley DTS-10
Blu Ray: Panasonic DMP-BD655