Depending on the wattage, you may need to go to more than 12 volts, though beware, as the DC resistance will be lower than the stated impedance.
Short term, you could exceed the RMS power by 50-100%.
Using a charged capacitor to deliver a kick, may be a way to deliver a good transient jolt without the problem of sustained power dissipation.
Another possibility, though it would probably need electronic switching, would be to bias the speaker the opposite way just before firing, so the excursion is doubled.
The other thing, the tube may be working against you, maybe funneling the narrow end somehow might improve things, as a flat stop with a hole is probably reflecting a pressure wave back.
Another approach springs to mind, if you could engineer it - flap valves and drive the speaker from an AC transformer, so it works like a gaint sized vibrator pump.