In the case of the VTF series subs from HSU, you have the option of plugging one of the two ports. To get the max output (highest SPL) out of the sub, you remove the plug from one of the ports and flip a switch on the amp. (not sure what the switch does). to get the deepest bass, you plug one of the ports and flip the switch to "max extension" mode. the trade offs are in max output mode, the sub doesn't goes to 25hz, but it plays louder. In max extension you lose about 4 db according to the manual, but the sub plays down to 20hz.


later,
simp