I thought all the higher powered car amps used boost converters, though running direct from 48v or more would be more efficient
They do. As for 48v, remember they want to mass produce these amps and the majority of users will only have one of them, if not, even smaller ones. All auto outlet centers are geared and knowledgeable with the 12v system and alternators and 12v batteries in cars. You don't want to make something fancy and efficient and cool if you can only sell to a fraction of a fraction of a fraction of users who would setup a 48v alternator in their car with 48v batteries.
Me personally, put me in the fraction of a fraction of a fraction of a fraction of a fraction of a fraction of a fraction of a fraction of a fraction of 1 percent as back in the late 80s & early 90s, I has a 120v inverter in my car and even a HiFi VHS with 8 hours of uninterrupted music on 1 cassette at basically full CD quality transferred from my home CD player, and it could never skip when playing back like car CD players of the time as well as me having to buy 2 copies of each CD I liked. It was kind of funny that I had a 'Sony' VCR remote in the frot of my car to select and play music. I used the VHS tape index functions to allow me to skip to any key album or son on the 8 hour tape.
Tell me, who here has seen or even used a Sony SLV757 VHS HiFi VCR in the rear of the trunk of their car for hiss free perfect sound 8 hour tape playback with seekable indexing?And yes, when I crossed the border to the US as I used to on many road trips, you bet the odd time they looked in the trunk, I was questioned and had to demonstrate it was my audio playback system.