Well I did research this and look at John Atkinson's data. The cross from that ABR bass unit to the mid units is at exactly 150 Hz and fourth order 24db. per octave high and low pass. The instruction manual says to set the receiver to large for the connection to the speaker terminals which are from the left and right outputs. So this needs a full range signal to those terminals. So in that case there is a passive fourth order high pass filter to mid units most likely, alternatively there could be a second order high pass passive and an acoustic second order roll off of the sealed drivers. Either way there is a 24db. per octave high pass.
So that is why the receiver must output a full range signal to the speaker terminals on the back. So the speaker set up has to be LFE + main. The crossover to the sub output should be set to 150 Hz, and the crossover on the subs as high as possible. This should hopefully give a balanced fourth order 24db per octave high and low pass filters at 150 Hz. That will be as closely as a separate sub unit could be contrived if he does not want to repair the amp. How well this will work is open to conjecture. Those speakers in fact measure quite well. There is however some crossover gain at 150 Hz and some degree of peaking.
The unknown is how well his subs function to 225 Hz. Since the cross is at 150 Hz, and the order fourth, he needs good performance from the sub at least half an octave above 150 Hz which would be 225 Hz, ideally an octave above which would be 300 Hz.
His best solution is to get the amp repaired. He has to test the sub driver in that defective speaker and report the DC resistance to DefTech, otherwise he gets no warranty.
The number may be out of the ordinary, as DefTech are known for highly unusual impedances in their active woofers and subs, often in the 75 ohm range. So it is anyone's guess what the DC resistance should measure.
I think the OPs best options are either to repair the amp, or ditch the speakers and replace. Those amps are miserable class C units.
