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You could keep the sub you have already an add one SVS, 700 euros should get you one or maybe a JBL LSR310s will be enough? One LSR310s might not seem so impressive, but when you have another sub already, place them well and EQ the response, that might be enough output?
It's incredibly easy nowadays to buy a vintage speaker and just turn it into a bomb ass gorgeous subwoofer for really cheap. If you're lucky you only need a drill to replace the driver with something with a low Fs and higher Mms.