Unfortunately, no. If you only use the XLR input on the 789, then you will not be able to use the SE line out. In other words, the SE line out will only output whatever is being fed to the SE input.
Just for curiosity's sake, though, I used both the balanced and unbalanced outputs from the SDAC Balanced, and fed both right into the 789. I confirmed I was able to get the same signal on the 789, playing simultaneously, with just a flick of the input switch in front. (Of course, the obvious difference, being the ~6 dB gain between the two inputs.) Also, since the unbalanced connection was present, naturally, the SE line out continued to work, feeding my STAX amp - regardless of which input was actively being used through the 789 itself.
While I wouldn't necessarily recommend such a convoluted setup to anyone (because you might as well feed the unbalanced signal from the SDAC-B directly to the Atom instead), but the concept does work - I just can't think of any use case to support its existence.
