@mofojo,
I know this thread is old, but just in case you never did contact Lyngdorf directly, I can tell you that in 2023 I did so for the exact same reason you started this thread.
You missed nothing and were/are at the end of the road. Haha.
I’m wondering if you disconnected your subs to compare the 3400 alone v 3400 + outboard amp on your main speakers. The outboard amp would have to be so much better that it overcomes the disadvantage of losing the short signal paths and the fully-digital nature of the 3400.
I tried it repeatedly over a years time and ultimately concluded the 3400 alone was much better. Of course with the sub integration via the app factored in, forget it.