open baffle that I’m experimenting with
I reread your post, and saw that you are using OB. In addition to the theorical -6dB extra rolloff due to cabinate diffraction (that you get with any baffle), you will also need to add a lot of bass boost 12dB/oct to compensate for cancellation from the rear wave below wavelengths
equal to longer than the baffle width.
You can apply active EQ with Roon to boost the bass to correct BS and OB cancellation, the limitation being the excursion of the driver. Bring a high sensitivity, full range driver, it has limited excursion. Pushing the driver to play lower and louder requires longer cone excursion. If you exceed the excursion limits of the cone suspension, you can damage the driver and it sounds more distorted the longer the excursion.
So yes, the answer is a helper woofer. If you are going to be powering this with it's own amp, then almost any large woofer will work, but you will need a low pass filter to cut it off where the TB comes in full. Since the OB TB has about 18dB rolloff due to BS and OB, you'll need similar rolloff on the woofer to blend.
There's a lot of details to consider to make it blend well in amplitude and phase. Type of amp, size of woofer, type of low pass filter, driver recommendation, Q, senstivity, ect. Gotta run off to tennis tourney, then car repair, F1, NASCAR, Patriots and pot roast dinner. But I'll write about it asap.
One good way to go if you can afford it is the rytthmik OB subs with active amp. That makes good powerful detailed bass, fully engineered to work as you wish. It's not too expensive, but not cheap.
More later
Rich