Power amplifier with step up converter
I'll go for it: Of course it is possible to design a step-up converter; let's, however, give the numbers a quick spin:


Max assumable audio power that you (see comments) aim for: 2·80 W + 2·60 W = 300 W
Class AB wild guess efficiency estimate: 50%
Necessary output power of your converter: 600W
Cost of that converter, size, cooling, weight: meeeeeep, not desirable.


In fact, whereas websites like TI.com usually have a good calculator for supply designs, a 12V in, 24 V @25A (==600W) out design is nothing they can offer - you'll have to design your own regulator for that, with beeefy switches. Oh, and make sure you have a > 50A capable fuse and cabling to that step-up converter!

I agree with Jaka, 20W per speaker should really be plenty for something that is designed to make it loud on the inside of the car, not the inside of a whole club. If you really aim for powers higher than that, I'd go for one of the hundreds modern amplifier chips that were designed for exactly the automotive market, and work directly of the 10 V – 14 V input range, at high efficiencies. 

Buying a single quad-channel Class D amplifier IC and redesigning your amplifier will be cheaper, less hot, easier to design &amp; implement, more automotive-safe (big step-up converter coils do not like being subject to constant motor-induced vibrations) and smaller than a design based on four Class AB TDA7294.

It really depends, but for a circuit that comes close to 95% efficiency (like, let's say, the TPA3245) under >10W loads, using a step up-converter becomes easier, because a) you don't have to provide power just for it to be converted to heat and b) because the class D architecture itself is pretty much immune to switching noise (as the output stage is needed to filter the class D switching noise, anyway). Still, I'd avoid it and, worst case, use four TPA3245 (or similar), one for each speaker, at raw battery voltage.

Most of these ICs can be used in a configuration where the FET output stages can be paralleled before the LC filters. For examples, see TPA3245 datasheet.