Horizontal vs Vertical biamping

Bi-amping with internal XO is a waste of time in either direction.

See Active Vs. Passive Crossovers (sound-au.com)

Digital XO from the source can be stellar if you know what you are doing.

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.

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?

You could leave them in the box, but they should be disconnected from the drivers. You could bypass them, but the possibility of screwing up is not zero. In the case of the Legacy SE, the speakers should likely be tri-amped as there is an XO between the mid and tweet. If done properly, the speaker was voiced to compensate for driver/XO interaction.

Something like the Dayton is pretty basic and the A/D is probably quite audible.

XO & D/A should be implemented in the digital domain. However, doing that properly is not trivial for what looks to be a fairly complex XO in the SE:

Not only that, but it's my experience that while the physical XO Hz & order may be known, they are only a starting point. Better performance may be obtained with active XO Hz & order quite different from stock params.

Not sure your Legacy’s would see the same dramatic improvement.

Read the active vs passive XO link above.

With amp connected directly to the driver, the amplifier is in total control of the motion. If there is ’iron’ and ’paper’ in the circuit, the driver sees a very high impedance at the XO Hz and is "flopping about like a fish out of water" 😉

Analog XO like those mentioned are still compromised, just like a passive, in that they have fixed orders and no delay.

IF one listens digital sources, DSP makes so much more sense:

• time alignment
• infinite slope capabilities
• driver anomaly correction
• room correction

dbx and the like are inflexible with fixed orders. On some speakers the drivers lose their blend and sound like individual drivers in a box.