Possibly another work around is to explore DB Keele 6th order paper on his web page and run the numbers.
My estimate is that depending on the QTS' (between 0.26/0.29 with some Rx from crossover L) an F3 in the high 30's is possible. Add a modest room gain and you are in business.
I consider adding significant Rx un suitable as you trade un warranted losses in driver efficiencies for extension when electrical boost of 6 db and Q equal 2 with Keeles formulae can offer extension in a WaF friendly box.