If coax can only support 3 tuners, then Direct's Genies wouldn't work either as they are single coax solutions also. I see no technical reason that they couldn't add a couple of tuners to the Hopper.
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Three is the max with the tech Dish is currently using in a typical install. Dish can obviously change their tech to accommodate more than three, but at a cost. Given that most Dish subs are now serviced by receivers other than Hopper would seem to indicate there isn't much current demand for super tuner systems, or at least not enough demand to warrant incurring lots of additional costs. Most subscribers have no use for lots of tuners. Want more tuners, add more Hoppers.