Sure Stehno, I'll tell you what I can about my trials and tribulations regarding that topic. I have built quite a few different speakers in my day, and the designs varied based on my knowledge at the time I made them. Good low bass isn't hard to get, it just requires the introduction of other design parameters that can be very problematic to the rest of the design. Most people would say that direct radiating bass from a large cone driver with a proper Fs and system Q is best for deepest bass. Vented or unvented propose different quandaries: vented usually creates notches or cancellation effects at some points, unvented tends to raise the Fs and limit low extension( large boxes and isobarik helps this problem, but adds others.) It's a "pick your poison" situation. TL(transmission line) can be very good if you get it right. Alot of tuning. To be short, this requires multi-way since no one large driver goes to 20k(or near). You already know my feelings on that. My solution was to address the problems created for the system by impedance spiking at the Fs(resonant freq of the driver). At this point, either a crossed-in sub or rolloff in response is the normal choice. The rolloff is caused by the increase in impedance creating a load that is usually 9 to 24 db down for the same power input level. It spikes around the Fs and tapers down below the Fs. By paralleling a properly calculated resistor(non-ind) across the speaker terminals, this spike can be reduced by about 60%-70%, thus allowing the amp to "see" a more driveable load with consequently reduced rolloff. This evens out the response curve in the most critical area - Fs and below. Now the amp can drive the speaker to much lower frequencies(+-3db), with rolloff ocurrring much deeper than before. Now, with my current one-way system, spl is limited by cone size and won't move the air necessary for real low bass. So I tuned a transmission line for half the Fs to bolster the additional octave of bass response(83Hz-41.7Hz). This is a big can of worms to tune without messing up the lower mids. Since it is a front firing line, notch effects do happen, but I reduced it down to slight by tuning. This impedance adjustment w/resistor also corrects the high rollofs as well, since they are also due to impedance rise, although at a much slower slope. Both ends improve, in my speaker's case, to within the 3db down point of the amp with regard to impedance. The price to pay is that the nominal impedance of the system is also reduced by some 25%. Some amps may not like this. However, you'll notice that the peaks are improved by a far greater amount, due to the product divided by sum nature of the load. It amounts to a very simple way to flatten and widen response of the interface between amp and speaker, with a minimum of additional complexity. Just for fun, go look at some driver graphs where they plot frequency response and also have a graph for impedance curve. You will notice that on a good driver, the flat section of the response curve is also the flat section of the impedance curve(around nominal). You will also notice that the rolloffs in the freq response curve also correspond to the rises in the impedance curve at the same frequencies. By flattening out one's rises(impedance), you also flatten out the other's dips or rolloffs(freq response). How about that?
