Horizontal vs Vertical biamping

When multi-amping with active XO, you  M U S T  remove the passive XO components from the speakers.

Regarding deltas between horizontal and vertical, there are multiple possible combinations of amp channels and speaker drivers in each configuration. The tube combinations of 2x ST100 are astronomical. Adding in OPT, driver and passive XO part tolerances, there are enough combinations to have a different system every day for the rest of time.

Look for a bryston  10b crossover.  

I have not rolled tubes in the amps yet. These amps are the “dark” version of the st 100s and they come with some pretty nice tubes. 

Why do you have to remove the passive components from the speakers, don't you just have to bypass them and can still leave them in the speaker cabinet?

When going active, you are adding another preamp circuit (the active HP/LP filter) so all the same quality variables come into play as with choosing a preamp. That said, modern DSP units are pretty amazingly transparent in my experience.  Wet your toes with the Dayton Audio DSP-408, which is versatile enough for home or car audio, has parametric EQ, and costs close to nothing.