Welcome to the world of system tuning and DSP!
There isn't much to argue on the fact that speaker building is an art requiring time and understanding of acoustic theories. While some simulation software may help for the design of your enclosure, audio measurements are the basis for any evaluation of your system. No matter how pretty your box is or the choice of high specs drivers, your "system" (i.e. enclosure + speakers + crossover) may very well have poor specifications if it isn't properly tuned (equalization/time alignment).
Thanks to the partnership between the miniDSP platforms and the well known freeware Room Eq Wizard (REW), miniDSP is much more than a digital audio processor. By providing a complete measurement + system tuning solution fitting in the palm of your hand, the miniDSP product line innovates once again, yet keeping our price very competitive.
You will need:
In the miniDSP interface, disable all equalization and crossover filters on your subwoofer. Turn off all amps except the subwoofer amp(s), or disable the other outputs in the miniDSP interface. This is so you can measure the "raw" subwoofer response.
Then, in REW, run a measurement sweep from 10 Hz to 300 Hz. Ensure that your SPL is adequate to get a clean measurement.
From the REW main screen, open the "EQ" tab.
Set the parameters as shown in the following screenshot. Note that in the Equaliser section you will need to select "MiniDSP" for plugins running at 48 kHz, and "MiniDSP-96k" for plugins running at 96 kHz.
Next, click on "Set Target Level" to set the target level to REW's recommendation. (You can manually change it later.)
Then click on "Match Response to Target." REW will generate filters to optimize the response and display the predicted response (response after correction). Here is an example with the measured response in dark blue and the predicted response in light blue:
You can see the filter settings calculated by REW by clicking on the "EQ Filters" button near the top of the screen:
In this example, only three filters were needed. If REW uses a lot of filters or uses filters with very high Q (> 20), experiment with the target level setting to try and get a smoother correction curve. You can also experiment with the flatness target and the max boost settings to get the best predicted response.
Once the predicted response is satisfactory, click on "Send Filter Settings to Equaliser" and save the settings as a file.
Then, in the miniDSP plugin, navigate to the Parametric EQ block that will be equalizing the subwoofer. Set it to "Advanced" mode and click on "Import REW File." Select the file that you just saved, and the plugin will then display the correction filter:
Confirm your result by running your measurement sweep again - it should be much smoother than before! Here in light blue is the new measurement, compared to the original measurement in dark blue - as you can see, the measured result is almost identical to REW's prediction:
In the crossover block in the miniDSP interface, re-enable the subwoofer's low pass filter. If you have a ported sub, you may wish to also add a high pass filter to protect the sub from over-excursion at high levels and low frequencies.
Then enable the other outputs (main speakers or drivers) and run full-range measurement sweeps. If there is a bump or dip at the crossover point, try adjusting the crossover settings (frequency, slope, phase) and the time delay of the subwoofer to flatten it out.
Once you have mastered the above procedure, you can improve your system even further:
You will now have the smoothest bass response you could imagine! Don't forget to ask on the miniDSP forum if you have questions.