Les,
As I begin the Model 19 build journey, i also have questions.
When you say bi-amp , are you using a DSP based or other "electronic crossover"?
If so, I'm not sure you are hearing the smoothest response the 811b can provide. If I understand correctly, most loudspeakers need a bit of massaging to their mechanical response to sound their best. I'm not sure an electronic crossover will get us there 100%.

During a previous build (GR Research LS9), I spoke to Danny the designer about bi-amping using an electronic crossover. He said there was a couple of circuits in the standard passive crossover that could not be duplicated electronically. Then tons of engineering techno babble went over my head. I should have taken notes.