You are so lucky. I have fished out of Kona a number of times and the short run to pelagic fish is amazing. We have to run three hours to have similar conditions here in New England.
Since you are not too likely to be looking for anything on the bottom in 6,000 feet of water, I am not sure why you would feel it necessary to have a 3kW low frequency sounder as you will get with the DI-FFAMP and R599LM. My view if I were in your situation would be to install paired B175M and B175H 1kW transducers that can be connected and run directly by the TZT3s. Then you would set the depth range manually too the upper portion of the water column, which is all you really care about seeing. Instead of spending the money for the D-FFAMP and R599, I would get a DFF-3D and mount a B54 transducer in a pocket on the keel. The DFF-3D will allow you to see bait and fish out to the side several hundred feet and down 100 fathoms or so. I find it really helpful trolling for tuna.
Just my thoughts, of course you may have other reasons for wanting the R599 though the amount of detail you would see in 1000 fathoms is very limited. I have a friend who has one in his boat. Normally we are fishing at the offshore canyons in 1200-1800 feet of water but if you keep going a few miles, it drops down to 1,000 fathoms. His sounder will read the bottom in that depth but it's really just a line without much useful to see.
Perhaps other West Coast folks who are used to very deep water will have input as well. The Furuno guys are based out west and will no doubt offer good advice based on what customers have told them.
