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I remember a mate of mine had an old Nova SR (which had an amp metre / battery level guage in the dash) with a HUGE amp and sub in the boot.
When the turned the bass too much up the amp / battery guage did a merry dance and the lights on the dash etc all went dim / flickered too. That was down to his alternator not supplying enough juice to the battery.
So, yes, I'd suspect the alternator and get it checked out as said above.
Currently in dave_mezza2006's garage:
2014 Vauxhall Astra Limited Edition 1.4 16v VVT Turbo