I see SH’s point. If my speakers have a -12db/octave roll-off starting at 60-80Hz, and my crossover is has a –12db/octave roll-off at a similar frequency, my effective acoustic roll-off will be –24db/octave. In order to have acoustic continuity using an –24db/80Hz crossover, a full-range loudspeaker would need a rating down to 40Hz.

One method sometimes used to help fill in a gap in a situation where there is a 80Hz crossover point used with a full-range loudspeaker not useful down to 40Hz is to build a mild custom boost into the crossover, as in a Linkwitz Transform Circuit, to the lower end of the desired frequency range. This helps compensate for a combined roll-off of crossover and loudspeaker, but is usually not something that can generally be applied. The designer must know the characteristics of the loudspeaker in question before determining what frequency and amplitude to use in such a compensating circuit.

In general, it’s better to have speakers where such compensation is not needed. Also, if the speakers performance is too limited, too much compensation leads to distorted performance. I suppose that such compensation only works when the assistance the loudspeaker needs is only mild to moderate