Hmmm... I suspect your most direct path to more volume and bottom end might be a bigger amp. I don't know what model Behringer you have, but almost all amps rate their power at 4 ohms, which means you're running probably closer to 150 W into a single 8 ohm cab. That's not a lot of juice.
Your current cab might be all you need with a larger amp that has more headroom and more low frequency current on tap. If the amp you choose has better tone control, you can better dial it in without having to add a bass extension cab.
If you can list the models of the cab and amp, more people will probably weigh in on which way to go.
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