Although I don’t do car audio, I typically tune my amplifiers with a low impedance resistor as a dummy load on an oscilloscope.
What wattage your amplifier is capable of determines how much your load resistor needs to be capable of handling. Although, putting resistors in parallel halves their load and therefore you can use 2 50w resistors, however in parallel you also halve the load (2*4 ohm in parallel is 2ohm load), so you can either use higher ohm resistors, e.g. 2*8ohm, or use 4*4 ohm and have them set up so two lots of 2 resistors in series, and then put those 2 groups in parallel with each other. These are just two examples to explain the concept, many combinations are possible. It is also important to have the resistors rated higher at higher wattages than what you’ll be using them for so you don’t get inductance from heating which might make the results unreliable. This can be helped by screwing them into a heatsink with thermal paste as an interface.
You then supply the amplifier with a clean sine wave (typically at 1khz).
When analysing the oscilloscope you will know when gain is too high since the peaks will start to flatten, when that point is reached I usually back it off a bit to be sure.
It’s also useful for amplifier testing since the fuzziness of the sine wave will demonstrate visually which amplifier might have a better SNR at a given frequency.
Hopefully this is what you meant by amplifier tuning Andy, otherwise this will have been a bit of a waste of time writing out this essay🤣