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Does that mean that there's no practical difference between the STF and the VTF?


The difference is in the amp. With the STF-3 in 18hz tune, it will have a rise in response down low because the amp does not have an EQ switch for it. With the VTF-3, flipping the switch to 18hz tune will give you flat response to 18hz.
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The VTF-3 also has the switch which changes the EQ and filtering to match a one-port configuration, so you get flatter response. I think it nudges the subsonic filter cutoff down a bit as well.


oops...got beat to it.