Like kosmo mentioned the 958 has dedicated DI crystals for down imaging where the 998 has computer generated DI images from the info received in the SI crystals...

Both the 958 and the 998 also have 2 separate 2d frequencies...with Switchfire
The 958 has:
200 kHz / 25° @ -10db
455 kHz / 16° @ -10db

The 998 has:
83 kHz / 60° @ -10db
200 kHz / 20° @ -10db

Rickie