dB Backline 915 troubleshooting

So I've been given a DB Technologies BackLine 915.

It's broken so that why i got it for free, and i thought it would be worth fixing. So when powered, and with a music input -> "line in" or "din plug" the tweeter works, but the 15 inch main driver does not.

I've so far disassemble the speaker, and removed the Amp (Digipro G2 600D). For testing the tweeter and main driver i hooked up a function generator, where i set in a sinusoidal from 5V 100 Hz to 2kHz. Here both speakers seemed to work. The tweeter got quite loud, so i've lowered the voltage, but the main driver were not that loud (Which is to be expected since it's stated to do 100+ Watt).
This was done directly to the speaker without the amp. (amp disconnected from the speakers)
I've heard about the battery testing method, but some say it's not healthy.

I have no experience with troubleshooting amps and such.

By looking at the PCB, no component looks to be blown or broken, and i have been unsuccessful of finding the schematics.

I have really no idea where to start the troubleshoot, but I've read that if i put in a sinusoidal waveform on the input.

I've thought about placing a new Class D amplifier inside, if it's beyond fixing.

I've included some pictures of the Amp, and datasheet.
 

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