You are thinking about it backwards. It is not a gain knob, it is an input attenuator.
Fully clockwise isn't max gain, it is zero loss.
What does winISD say the max SPL should be, given this driver and the box you put it in?
Do you have a UMIK-1 and REW to confirm the SPL?
Furthermore, you can get a multi-meter from walmart or any hardware store, you can disconnect the speaker wires from the driver and connect the multi-meter directly to the cable ends with the multi-meter in VAC(Volts AC) mode. (Don't let the wires or probe ends touch or the inuke will die from 0-ohms.)
Make sure all EQ is off in both the AVR and the inuke.
Play a 60hz sinewave via REW, it must be 60hz.
As per ohms-law: 300w @ 4-ohm is 34.6 volts RMS
You should be able to hit that.
It is likely just an input-signal issue.
There is a gain textbox on the limiter tab in the inuke software if I recall. You can try to crank that if you actually want boost. All that will do though it make the amplifier clip more easily.
If you are already clipping it, then that is all the SPL that wattage will produce on that driver/box, if you want it louder you'll have to turn the limiter off or set it to max-power. That of course will be at your own risk, you may end up smoking that driver from excessive power.
There are two things that cause a driver to die: heat and bottoming. Power causes both of those.
Heat and low ohms or excessive current or undervolting at the wallsocket can also kill an amplifier.
In general, sinewaves put a lot of stress on the driver and amplifier. Pretty much maximum stress. If something is gonna melt it will be from sinewaves. Mid-bass causes the driver to park and boil, and low tones causes bottoming (and sometimes also heat, especially with single digits.)
Clean subs will sound less loud, that is because distortion+SPL will always be louder than just SPL.
Also, humans hear distortion as loudness. Your kid's iphone isn't loud, it is just playing music you dislike (with distortion). It's only 90db worth of treble and zero bass. (Therefore: go to bed!
Annoyance can be perceived as loudness too, 3 minutes isn't very long, but all the people on your block will be hella annoyed after just 2 minutes of basshead music. It literally slows down time for them, ~2 minutes feels like ~2 hours when the noise is undesired and unwanted. (Let the waterboarding commence!
In general though, 12's aren't very loud, even at max power and max excursion.
That said, loudness is relative to the person, even a single 18 is quite to someone who is acclimatized to years of usage of an array of subwoofers and an array of amplifiers.
It could also be that you are just a basshead, bassheads are never happy with any amount of bass. They will adapt, like the Borg, and then just desire MOAR...
and THAT is how upgraditis is born.
You are just slowly coming to the realization that arrays of these products are REQUIRED.
Get out, while you still CAN...