Make sure you have these wired @ a Final 2 ohm load and not @8ohms in the sealed enclosure
Subwoofer Wiring Diagrams for Two 4 Ohm Single Voice Coil Speakers
www.the12volt.com
Makes sense. You may be right on the gain; 2 subs on 300 watts seems anemic.We know how enclosures affect subs and it's especially predictable with sealed enclosures. Just because there is an obvious mismatch between the enclosure and subs doesn't mean that's the cause. I'd suspect cold solder joint at the terminal, bad wiring, a VC on it's way out, maybe an issue with the amp.