Just to clarify on my previous statements.
I think that your analysis for the series RC is not right. Your analysis is only effective if the RC circuit is in parallel with the load wherein it functions as a low pass filter. When you have series RC, your R just forms a voltage drop (for leading edge peak voltage reduction) and your C is effectively shorted when there is increasing frequency.
>> Above refers to the RC circuit.
As you mentioned with F3 from 98 to 147Hz you can think of the C as a short starting at that frequency band. If the input resembles a square wave, the voltage formed at the C looks like a spike then when it achieves peak voltage it ramps down. Of course with higher C the ramp down is slower.
>> Above refers to the larger C.
Without the R the peak voltage is higher. The resulting voltage is a dV/dt or a differential. This is unlike that of a low pass filter where you have an integral or like a ramp.
>> Above still refers to the larger C without the snubber resistor so the 220nF snubber capacitor is now in parallel with the larger C resulting to 135.22uF.
Guys, smaller speakers have higher min frequency operation so 100 - 150Hz should be fairly acceptable. If you want to try it with bigger speakers use higher value capacitors. Further, it doesn't have to be increasing the value of the capacitor you can also parallel a smallish value choke which should effectively end issues on low frequency performance
My point is rather than throw questions try the circuit first then ask questions later. You can make it external. If it works for you then fine. If it doesn't you can play around with the snubber resistor and capacitor values. There are ways to make it work
Also, you can PM so I can calculate it for you
The snubber circuit posted here is just a guide for you. You can be creative and add your own circuit or components. You do not have to be limited to what I have posted