Electrical – pam8403 using low wattage speaker

It might be slightly less bad but the ratio between maximum excursions is only 1.6, giving you 4dB more (potential) volume - barely perceptible.

You'd be better off either moving to a larger radiating area - a bigger driver (a 4 inch woofer is an oxymoron!) - look for one whose output doesn't roll off before the bottom of the band of interest.

Or if you can't afford the space, perhaps a tuned port to augment the output around 100Hz. Port tuning can be tricky, but there are loudspeaker enclosure design programs that will help. You're probably looking at an esclosure volume of about 4 litres with a fairly small diameter (25 or 30mm) port as a starting point.

Alternatively : is the speaker being driven outside its linear range by frequencies below the band of interest? Perhaps a high pass filter set to 80Hz will pass all frequencies of interest but protect the speaker from high amplitude 20-50Hz signals that can only drive the cone out of its linear range and potentially cause damage.

You can't just throw any random speakers together and expect a good result. Each speaker has its own impedance curve which can vary dramatically with frequency and has to be taken into account when designing the crossover.

Also their sensitivities are probably not matched so you could need padding resistors to balance them, and the tweeter usually can't handle as much power so it needs to be protected against getting too much at lower frequencies.

I haven't done any calculations on you design, but it looks iffy to me because even below 12KHz the capacitor partially shorts out the drivers and puts high power into the tweeter, and the amp might not like the low impedance load.

The two larger speakers should be fine in series provided that they are identical. If the original circuit is correct it suggests there may be some difference between them - or perhaps L1 is wired like that to modify the impedance and/or amplitude curves.

The main difference between that design and yours is that it uses the natural inductance of the drivers as part of the high pass filter. Since driver impedance increases at high frequency this transfers power to the tweeter without increasing load on the amp. However without a full analysis with all speaker characteristics taken into account it's impossible make an accurate calculation.