FWIW, historically below 60 Hz was sub-bass based on musical instruments till THX came along and designated 20-80 Hz for cinema sound and 20-120 Hz for small cinemas, HT and have since added ever higher cutoffs for ever smaller/lower power consumer systems, so nowadays have to define based on system design, i.e. one person's 'sub' can be another's bass or even mid driver if > 250 Hz based on musical instruments.
If the speaker impedance go below 3Ω, even below 2.5Ω, except cars and PRO amplifiers, the HiFi one's are not happy with an impedance so low.
Why do you say I don't understand the concept? It is a kind of perpetual motion machine that puts out more energy than it receives? The gain on the bass region is obtained by drawing more current due to the reduction in apparent impedance in that region because of resonance.
The impedance does not get lower than without the capacitor. This is impossible. So the amp does not see a lower impedance and is not "destroyed" Think about it. You got that wrong. The impulse does not suffer, too, if that the next thing. Look how many caps you put in front of the tweeter, or any impulse must be dead after your x-over.
