I'm guessing your other Lopi had burn tubes, unlike the rear afterburner of this model. Though you may have a gasket problem or an afterburner problem, could just be the nature of your flue setup and that stove, with its downdraft burn technology. How tall is your flue? A tall flue, especially in strong-drafting cold weather, is going to give you shorter burn times.
Although some people can get very long burns in a downdraft without excessive temps (especially with the Harman TL300, it seems), other owners of the Leyden/Arbor, the Oakwood, and older VC Everburn models have had ongoing problems that border on overfire... and the shorter burn times that go along with that.
Some have tried reducing the draft with a pipe damper, some have tried modifying the secondary air supply (a modification which might void the warranty), and some have traded in their stoves. I have tried putting some foil in the air supply, but then I find it more difficult to get the secondaries to kick in in the first place.
Have you taken flue temps, or temps on the rear of the stove where the AB is? (I don't know if that is possible on the Lopi, but it is on my Oakwood.) As long as temps in those places are not excessive, your problem would seem mostly limited to the shorter burns and lower efficiency you're getting. The inability to dial the stove down is not so bad in the bitter cold, but it's a real aggravation in the shoulder season.
