My theory with that was as follows. Say, in your room, the STF-3 may be able to do max, 103 dB @ 40 hz, 103 db @ 30 hz, 101 db @ 25 hz, and 98 dB @ 20 hz. Even though it has strong output at lower frequencies, it is still rolled off from what it can do at higher frequencies, so they advertize it that way. Perhaps a spec saying how far down all of their subs are at 20 hz in addition to the 25 hz spec would be better.