class D with car power supply

Sure Electronics (now called Wondom I think?) do a single board SMPS for car use. They do a 300W and 500W version with switchable outputs from 24v - 48v in various discrete steps. They are a basic TL494 circuit but seem quite reliable in my experience and the output is nice and clean, I do not know what their EMI radiation is like though.
 
Excellent reco thanks!

Really a DC-DC boost converter, "SMPS" implies AC input.

outputs DC24V, DC30V, DC36V, DC40V, DC48

WONDOM | STORE

500W Boostmodule DC to DC 12V Output 24V 48V Switching Power Supply Board Wondom | eBay

Any idea if it has good enough current limiting to charge a LFP bank?

There are also devices designed as solar controllers for higher voltage packs, Genasun is a good one,

cheap chinese one is MPT 7210A.

So e.g. LFP 16S for 48V nominal would give clean output for the amplifier, high currents on tap for peak transients

While the DC converter/charger keeps the bank topped up from 12V.

With a big enough bank, the (maybe noisy) input device can be turned off for a critical listening session X hours long.