1) Bi-wiring is BS. Period.
2) "Passive Bi-amping" is BS. You’re still delivering a full range signal to both the LF and MF/HF passive crossovers, the unused half of the signal is just turned to heat.
3) Horizontal Bi-Amping allows for hybrid amp usage e.g. SS Bass Amp / VT Mid/Highs. If the concern is power supply demand, that’s an amp issue; bi-amping has no impact, per se. Get a bigger amp if needed.
4) Vertical Bi-Amping uses CH 1 of the first am for LF and CH 2 for Mid/Highs. In theory minimizing incongruities dues to differences in amp design.
ALL bi-amping requires a line level crossover to provide LP and HP as well as level matching functionality.
@knotscott said "Most active crossovers can’t address problem areas within the drivers like passive crossovers can. They only act as high, low, or bandpass filters, no notch filters, no shelving networks, no zobels, etc. Hook an active crossover to a driver with issues, and you can have a mess that can’t be compensated. A really good passive crossover with high quality parts can sound amazing. Passive crossovers can be more complicated to design well, and many use cheap parts that effects performance." This is by and large true. There are two mitigations however: use a DSP that can provide notch and shelving as well as crossover functionality or connect the amp post-crossover and pre-equalizing elements. This preserves the compensations, but may not easily be accomplished with some crossover designs. I probably would back out at that point, unless the MF/HF access is provided where necessary by the manufacturer.
All that said, there are two major advantages of vertical and horizontal bi-amping done with an electronic crossover: Improved dynamic range, by as much as 6 dB. 2 - 50W amps can deliver the dynamics of the same speaker driven by a 400W. More realistically, a 75W LF and 25W MF/HF - which argues for vertical bi-amping. Active speakers like Genelec G-Series, KEF LS-50 and LS-60 Wireless or JBL 4305 and 4329 are a good examples of this type of asymmetrically powered active speaker design.
The other advantage is eliminating the both the LF series inductor(s) and their associated DC resistance, which destroys woofer damping and control as well as keeping the back EMF from the LF driver away from the MF/HF elements, reducing any distortion, and improving power handling. The latter two are easily audible.
It should come as no surprise that virtually all high-performance live sound systems are bi-amped, tri-amped or even quad-amped, and have been for over 50 years. The systems I co-designed and worked with in the mid-late 70s sprung from the Altec Voice of the Theater tradition, eventually using subwoofers bins under the stage crossed over between 80-120 Hz, LF boxes from there to 800Hz and large 4" compression drivers from 800- 6Khz both vertical line arrays, and 1" compression drivers from 6KHz up, all hung, flown from the venue ceiling . We went through a number of amps, beginning with the 150 W/Ch Crown DC-300A (not roadworthy), An Altec 200W/Ch 9440A (not reliable) before settling on the now-legendary Yamaha P2200.
Two final notes - the advantages proper bi-amping are clear: improved LF performance and dynamic range, but are most apparent only in high output (loud) applications. If your home listening never gets above 90dB or so, bi-amping is doubtfully a cost effective add-on . If you have a big system in a big room, it’s the only way to go.
Virtually all subwoofers on the market today are actively bi-amped, although few have proper HP filtering, relying instead on the ’Bass Management’ capabilities of the preamp or AV processor. Low end subwoofers often have speaker level connections to the subwoofer, then pass-through connections to the main speakers. This is a very economics-driven and compromised solution.
