Getting back to driver selection, perhaps the question should be restated a bit to provide guidance about selecting the proper driver. In an oscillating system, Q is a measure of the energy stored vs the energy dissipated. A higher Q system will be able to play at a higher volume, over a narrower frequency bandwidth. Increase Q high enough and it will be come a one note wonder. Not very musical but an excellent way to win a car audio SPL competition. Lower the system Q and the bandwidth increases, allowing the system to provide a more musical response, but at lower sound pressure levels.
Q is related to the damping, zeta, by Q=1/(2*zeta). In an oscillating system, when the damping is 1, the system will have a transient perfect response with no overshooting and no unwanted oscillations, but with reduced response in the lower frequency region. Decrease the damping to 0.707 and and the frequency response curve will have the maximum flat response with minimal overshoot and unwanted oscillations. Decreasing the damping below 0.707 will increase the response in the low frequency region but at increasing levels of overshoot and unwanted oscillations. In a speaker system, the overshoot will cause higher sound pressure levels at lower frequencies, but the oscillations will reduce the clarity of the sound. Thus, speaker design is a balancing act of choosing high enough damping to keep the musical reproduction accuracy, but low enough so that the bass volume doesn't get reduced to much.
A zeta of 1 for a transient perfect response corresponds to a system Q of 0.5, which produces a very tight bass but at reduced sound pressure levels. A zeta of 0.707 for a maximum level bass response and reasonably small overshoot and oscillations corresponds to a system Q of 0.707. Reducing the damping and increasing the Q to 0.8 or even 0.9 increases the low bass response and warmth of the bass. Increase the Q to 1 and beyond and the bass volume will increase but the bass will be mushy.
From my understanding, vented enclosures and folded horns are not really thought of in terms of overall system Q. However, system Q is what I'm most familiar with so I'll keep the discussion in terms Q. The overall system Q, Qtc, should be some function of the cabinet Q, Qc, and the driver Q, Qts. Thus, Qtc = f(Qc, Qts). The driver Qts is the parallel summation of the mecanical Q, Qm, and the electrical Q, Qes, and thus Qts = Qm*Qes/(Qm+Qes). Ported subs typically get good results with a driver Qts between 0.2 to 0.5. The resonance frequency of the driver, fs, divided by the driver electrical Q, Qes, gives a quantity called the Efficiency Bandwidth Product, or EBP. An EBP of 50 or less indicates the driver is more suitable to a sealed enclosure, while an EBP of 100 or more suggests a vented enclosure. An EBP between 50 and 100 indicates the driver could be used in either a sealed enclosure or a vented enclosure. I would assume that a folded horn will be similar to a vented enclosure, thus a folded horn should benefit from higher EBP levels, perhaps in the 80 to 120 range.
The auto tuba listed range for driver Qts is 0.3 to 0.5. I assume the overall Qtc is a directly proportional function of Qts and Qc. Thus, increasing the driver Qts should increase the overall Qtc. I would expect a Qts of 0.3 to produce more detailed bass, while a Qts of 0.5 would sacrifice some clarity for more volume in the low frequency range. Therefore, a Qts below 0.3 will tend to reduce the low frequency volume due to higher damping. Therefore, I don't really see any benefit to going below 0.3 for Qts. Is my reasoning correct? Will a driver Qts of 0.3 produce more detail while a driver Qts of 0.5 produce more volume? Will drivers such as the MCM 55-2421 with a Qts of 0.22 produce excessively damped bass with lower volume in the low frequency region? Or is my reasoning to simplistic for the complex behavior of folded horns?