Some find it very advantageous in the HB units of having 2d in addition to the DI...

HB 596c HD DI
Down Imaging® w/ DualBeam PLUS™
w/SwitchFire™
2d...
200 kHz / 25° @ -10db
455 kHz / 16° @ -10db
DI...
455 kHz / 75° @ -10db
800 kHz / 45° @ -10db
Depth: 250ft (DI)/600 ft.

Rickie