The idea of a Lite version of REW was only a suggestion and I cannot argue with your perfectly valid opinions. But do remember that there all levels of skill and understanding about subwoofer performance, the parameters involved and working with the most basic software.
It just seems incredible to me, in a demand driven world, that no simple LF filter box or basic test software exists for the average AV/HT fan to calm his subwoofer's in-room peaks. Perhaps they do but I just haven't heard about them?
Placing the burden for the test software on John's shoulders is of course quite unfair. Though his task would be made all the easier not with a more complex BFD but with a much simpler filter box of much more limited frequency range. Providing perhaps ten programmable (cut only) manually set filters in a single range from 10-200Hz. One phono socket in and one phono socket out with a small display to confirm the filter settings frequency, depth of cut and breadth. Reasonable audio quality is of course desirable.
Given what Behringer can do with £80-100 dollars retail in the BFD the sub filter box need cost no more than (say) $10.
