Makes some sense toothboy. When running in max output mode on the VTF-3 MK3, one will have wider dynamic range and higher output capability above 20Hz, which covers most program material. Max extension mode (whether via port plug or turbo) will give deeper bass extension, at the expense of output capability over the more common bass ranges. Note that one can also try running the sub with turbo, while setting the bass extension switch to the '2 port open - 25Hz' mode. This will give an overdamped response (ie. early rolloff compared to '1 port open' mode). It is physically safe to run the sub this way (but never run the sub with both ports open, and bass extension switch set to the '1 port open' mode).
Pete - Hsu Research