Hi all just resently built 4 USB with fane 18b drivers
If you stand them up right the drop to 48 hz which I was very happy with.
If you lay them flat they drop to 42 hz but was driving them very hard of a crest 10004.
What was very intresting tho was my Ukraine guru
Lex put, piece of 2" batten underneath back of the cabs Wich angled the cabs towards the floor
And got very close to 40hz but not very practical
We measured this using a freq scope with mic pic up some ex soviet
Piece of kit
Could not really say how accurate it was
They told me it came out a submarine.