420W TDA8954TH Overkill? Lithium powered project

I don’t suppose you could swap that speaker for a 4 ohm version of the same (assuming there is one available), can you?

It’s just that, in most cases, class-D amps (especially chip-amps) aren’t rated for very high voltages but are easily capable of large(r) currents, so lower-impedance drivers are kinda better suited.

Sure, there are chips like the TDA898xx series, but they need split supplies, as mentioned above.

For 150W into 8 ohms, according to page 33 of the datasheet, you’d need a +/-27V supply or thereabouts. Although for subwoofer use, i’ve got a hunch your SPL will be rather limited by the maximum excursion of the woofer at low frequencies (especially in a vented enclosure), rather than available power.