HB 999 DI Transducer Guidiance

Bought a HB 999 SI.
Want to add a DI shoot thru hull transducer for high speed.
Confused on what DI shoot thru fiberglass hull transducer to purchase.
Humminbird tech support says 710225-1 XIH 9 DI is the correct transducer.
HB Tech support says with the new software release the XIH 9 DI T is the optimal DI transducer for the 999.
Reading all the various forums suggest to use the 710164-1 XP 9 20 as the correct transducer.
What would you suggest for DI high speed on the 999 and the difference between the two listed?
Signed..... confused.
Thanks

I think you have your terms confused.
Down Imaging is just like Side Imaging and not anything like regular 2D sonar.

Typically you would pair a dual beam 83/200 kHz shoot-thru with a SI transducer with a SI unit. The connection cable is the AS SIDB Y.
The shoot-thru is the XP 9 20.

While you can use a transom DI transducer for shoot-thru, you would have to use a transducer switch to use the DI crystal in the DI transducer. Then choose to view dedicated DI or SI by changing the switch position. The switch is the TS3.