If bulk is not an issue, would not the best way to solve the high impedance output be to pair it with an amp with lower output impedance say for instance Alo National (what about Leckerton UHA 6 with optical inp?)?. I don't mind the bulk but really need to be able to drive sensitive IEM's and I'm no fan of the DX100!
Even with an amp, it is better to have your output device into the amp with the lowest output impedance possible. Tube preamps achieve this with a tube in the buffer position as a cathode follower, which can lower the ouput impedance to less than an ohm. And if you have to stack to get a lower impedance you are defeating the purpose of the DAP.