Not sure why downtube shifters and cranksets are related in anyway at all. Cranksets don't care how they are shifted. Downtube shifters, bar end shifters, STI, Ergo, SRAM's shifter. Friction or indexed. Crank doesn't care. I am not sure how modern new downtube shifters work. For the front derailleur anyway. Is the front derailleur shifter indexed with distinct clicks, stops. Or is the front derailleur shifter friction/ratchet based. This will affect whether your current front derailleur shifter can handle a triple or double crankset. If there are distinct stops, clicks, then its likely your downtube shifters are for double cranksets only. So your crank will have to be a double crankset. If the front derailleur shifter is indexed, clicks, stops, then you can't really stop in the middle. It won't click in the middle and will tend to want to go up or down. Nothing to hold it in the middle of the stroke. Now if your current downtube Dura Ace shifters work with friction, ratchets, on the front derailleur, then you are good to go. Push the lever all the way forward and you are in the smallest ring, no matter if its double or triple. And if you pull the lever all the way down, then you are in the big ring. Double or triple, does not make any difference. And if you put the lever in the middle, then you will be in the middle of the crankset. Middle ring if its a triple crank. Or making lots of noise from friction if its a double. Kind of halfway between both rings on a double.
If your question is about what crankset to use. Get one with as small a inner chainring as possible. If triple crank, then make sure the inner ring is 24 teeth if its a 74mm bolt circle diameter crank. Or 22 teeth if its a mountain bike triple with 64mm bcd. You can also use mountain bike cranksets with only two rings. Mountain bike double crankset. You can get a 22 tooth inner ring on those cranks. For high gear, that is never a problem. You only use the highest gear when going down a mountain with a tailwind. Now days many cassettes have 11 tooth small cog. So a 40 or 42 or 44 chainring gets you a 4 to 1 ratio or close to it. That is close to a 50x12 or 52x13 or 53x13 high gear on a road bike. How often do you use the highest gear on a road bike? Rarely. Unless you are going down a mountain with a tailwind. Only professional cyclists have enough leg strength to use high gears. Just use whatever big chainring the crank comes with. You will spend about 95% of the time in the middle gears, 4% in the low gears, and 1% in the high gears. Don't worry about the high gears.
Will XT parts work with Dura Ace? Yes. Crankset and chainrings and cassette cogs don't care. Shifters and rear derailleur, maybe the front, kind of care somewhat. And try to match up the front derailleur with the crank. Triple front derailleur with triple crank. Double front derailleur with double crank. You can get away with mixing them, but its best to match them.
