It is technically difficult to make a single driver that covers the full audible frequency spectrum.
Manufacturers succeed to a greater or lesser degree.
The Lowther drivers naturally provide these characteristics and it is up to the cabinet designer to try and accomodate them so as to produce a speaker with relatively flat frequency response. The Lowther drivers have a broad frequency peak from about 1 kHz to 10 kHz. Cabinet designs need to increase the output below 1 kHz in order to match seamlessly with the frequency peak. Most designers do this by employing a rear-loaded bass horn. This limits bass output and creates a ragged bass frequency response.