I have a very similar hood. Without seeing a schematic it's hard to tell if this is helpful at all, but you can have metal fabricators make custom curved ducting and modify how the external vent attaches to the ducting running from the hood's transition. By "vent" do you mean ducting? Also, we have a modified right angle 8" round turn that is a "flex" transition, but not the accordian style--it is hard vent with adjustable interlocking sleeves that enable it to join the transition (top of the hood) to the ducting while maintaining the diameter. I have read that an issue with flex is turbulence from the rough surface can catch grease which can cause problems (along with noise and it loosing volume from collapsing or distorting). I know that VH claims full grease extraction into the blower box, but I've grease deposits on the inside beginning of my ducting and it sounds like this is where your flex ducting is. fyi, I also have a 36" AG Wolf.

One would have expected Prestige to provide some advice, but without it, I recommend you match six-inch vents with six-inch ducts. Larger ducting reduces resistance but also slows down the air velocity. Commercial ducting requires 2000 ft per min air velocity to limit the amount of grease that will condense. According to some sources I've read, residential velocities should aim for the 1000 to 2000 ft per min range. The two six-inch vents can be merged into a duct with the same cross-section, around 8-inches, but only if both power packs are powered at the same time. Otherwise, keeping the velocity up requires individual ducts. A call to Prestige may be desirable. kas

Answering the title question -- they often do but not always. Vent hoods are often designed to have one damper at the hood's exit vent and one on the outside wall. The idea is to limit hvac losses which can be a big deal in very cold and very hot climates. In some mild climates, installers sometimes skip the interior damper. Also, you have round 8" ducting going directly out the back through the wall? How thick is the wall? How far into the wall are the holes for mounting the interior damper? How thick is the wall? Will there be enough room for the damper to open fully? Sounds like you are in the middle of construction. You might have better luck in quickly finding somebody here who can help with direct experience if you can post the brand and model of your vent hood or maybe just start a new thread with the brand and model in the title.

building2017 wrote: ... - What are the silencers I have read about and how do they work/would they work here? These are like giant truck mufflers, except designed for various diameter ducts. They have low pressure loss (forget the value but it is a lot less than baffles). Fantech is the source. - Any benefit re:sound to using two smaller liners instead of one big one and only using one at a time when using only one side of stove? Or is this magnifying the cost for no good reason other than sound? (Because I'm assuming I'd then have two runs up and out and two things on the roof because they wouldn't combine to avoid a turn and to ensure proper function? If only one side were run at a time, then MUA costs could be lower, but where code requires MUA, I am willing to bet a donut that the AHJ will demand an MUA system that can handle both at once. In theory, the sound reduction will be only 3 dB less. Also, when using the central burners, you will still need both systems for capture and containment. Well, I guess three things on the roof then with the MUA. While commercial systems mostly use roof MUA entry, and I use roof MUA entry, the options for residences are wide open and may include side of house entry, or entry into a basement that has an air path access to the kitchen.