I know the 899 users who upgraded to the HDSI have been dissatisfied because of the issues with the SI 800Khz.
I think this has also been an issue with other 99 series units.

Here is a copy of a post on another forum.......Now if they would just fix that dang 83 Khz issue.... Sharon is in Humminbird customer service. If I had a 99 series unit with this problem, I would definitely be giving them a call while the unit is in warranty:

Originally Posted by TundraDrummer
I spoke to Sharon Bates today. The 800khz issue on the 899 is both a software and a hardware issue that they can now fix. She is sending me a shipping label in the mail that will not expire before I intend to send my units in by the end of September. She told me turnaround is 3-5 days. Call the service number and ask for her by name. 1800 633-1468