Wow a really nice looking hybrid! Wouldn't adding another 29689 solve the issue though? Did you limit the dynamic driver to bass frequencies only through the crossover? I'd think the RAB drivers sound nice or Unique Melody wouldn't be using them.
Can get my hands on some GR31587 to try out as some mid range drivers. Hope they sound nice :P
Did you guys see the Sony custom? Interesting how it uses a Sonion 2356 instead of its own BA drivers