Promise not to laugh?

I want to bag my plate amps (one broken, the other sucks) in a pair of subwoofers. I'll drive them with a stereo amp I have from the second outputs of my pre-amp.

I know I can easily build a circuit, I just don't know the terms or where to go for info. Tried a 2-way crossover designer, input 4 ohm woofer and 90 Hz crossover with second order (12dB slope) Butterworth. That much I know if it were a 2 way.

Can I just use the woofer part of the circuit and put a big, 10mH coil in series and great big, 312mF cap across? Can it be that simple?
that will work.
You will need some way to adjust the signal level input to the woofer amp.

And you might want to consider a line lever crossover between the preamp out and amp in.
Either passive line level or a simple 1 op amp active crossover. It will be a lot easier to adjust the frequency and slope if you want to try different options.
search esp crossover projects
Yes, yes it can...........In one 'experiment' I had a 12" DVC sub, built the enclosure @ 24 cu ft...!!! & a pair of 10mH high current $$ coils..........sounded great.
I promptly disassembled (busted up) this sub..kept the second one(Isobaric push-pull idea).......the 'original' is gathering dust.