If the voice coils of both speakers are burnt it is unlikely you have a manufacturer defect. And if it sounded good and clean in the beginning at a particular volume level the equipment is probably fine.
You simply melted the insulating enamel coating off the voice coil wires with too much power/heat. For the most part copper wire is copper wire and enamel insulation is enamel insulation. It's not a quality issue.
Power manifests in a number of ways. The Harpoon tests at 225 RMS watts average per two channels at 4-ohm at 1% distortion with a 14.4 volt supply. At any supply voltage that you are likely to produce on your boat that places the power just within the 175 watt thermal capacity of the JL Audio 7.7 M series. But with little to no room for error on that large of an amplifier. And in the case of the 7.7 MX series it is way too much power and a total mismatch.
However, an amplifier can put out a significant multiple of that power at a higher distortion level. And that can be where you have serious damage potential. It might have sounded clean when sitting still but when running across the water with noise competition and after a long day and ear fatigue you may not be a judge of what is clean any longer. People tend to continue to sneak up the volume as the day grows longer and when underway, when they can't hear distortion.
And just because the input gain on the amplifier was set low doesn't mean that another component upstream in the signal path wasn't clipping or compressing.
There is a big difference between 80 and 100 Hz HP. Understand that as the frequency lowers there is more signal gain to produce the same output. IMO, 80 Hz is way too low for a 7.7-inch. 100 to 125 Hz is more reasonable. 100 Hz for more midbass at moderate listening levels and 125 Hz for maximum volume and projection. Just depending on your usage. The tower version of the JL Audio M770 is a great sound quality speaker, and certainly a favorite of mine. But if you expect projection at wake range with one pair of these 7.7s then you have the wrong speaker.
Selective equalization boost doesn't sound like much of a volume increase but never the less a moderate 6 dB boost is four times the power. So at max volume any degree of EQ or bass boost is lethal. Keep all tone controls flat or turn down the volume by one dB of every one dB of boost.
Also, running at a low supply voltage isn't doing your equipment any favors.
I can't guarantee it, but your problem is likely described in one of the above scenarios. Set-up or usage.