Most speaker manufacturers us the 3dB down point as the end of a useful frequency range, unless otherwise stated. Also, sub maximum output may include corner loading, or give output for both corner and non-corner placement.

A quick bit of poking around on the 'net and people seem to think that DT exaggerates by using a 10dB down point as the low frequency point on subwoofers. If true, then it's apples-to-oranges when comparing low frequency extensions to other manufacturers.