I have the alpine kit and I did not experience any problems.
With the wiring diagram of your kit, it should be possible to bypass the amplifier in the trunk (boot) and drive the speakers directly from the radio to diagnose the problem. To do this you will have to make a custom jumper/pins connector. This is not too difficult if you are handy. It is easier than pulling out the Alpine kit from the car.
It should be easy to test one speaker at a time. This will allow to isolate the problem. Is it the amplifier, wiring or speaker?
Note: To do this test, the amplifier need to be removed and disconnnect the +12V fuse in the kit wiring.