sfall, I complete understand what you’re saying (except for one matter, that of you using a two channel power amp on each of your speakers, without a line-level xover), but you aren’t getting what I’m saying. First of all, the op stated he was looking for suggestions for a passive LINE-LEVEL high-pass crossover for use at 100Hz. I used upper-case for line-level to draw attention to the fact that the op is talking specifically about that type of xover. A line-level xover is completely different from the speaker-level x/o that you are talking about. Not only that, the suggestion that the op "Do a vertical biamp instead. You won’t need the xover." makes no sense, and for a couple of reasons.
First, his speaker contains no internal, speaker-level x/o with a filter at 100Hz! The op states he wants a line-level x/o to put between his pre-amp and power amp, to filter out the sub-100Hz frequencies from reaching his power amp and speakers. 100Hz being a very common frequency at which to add a sub, we can safely assume that is the reason for the need for a line-level xover.
Let’s define our terms: A speaker-level xover is the common filter installed on the inside of a loudspeaker. It receives the full-range (all frequencies) amplified signal from the power amp, separates it into either two or three parts (for a 2-way or 3-way speaker respectively), and sends the filtered signals to the speaker’s drivers. In this scenario, the power amp is "seeing" and amplifying all frequencies. Many speakers can be used in ONLY this fashion---the speaker’s one set of binding posts receiving the full-range signal from the power amp, the speakers internal xover then dividing the signal into frequency bands which are then sent to the appropriate drivers. The filtering is done with passive parts---capacitors, resistors, etc. These parts not only divide the already fully-amplified signal received from the power amp, they are also used to create the balance between all the drivers, the output level of each driver. The parts are also used to create the "order" of each filter---1st-2nd-3rd-4th, which is 6-12-18-24dB/octave.
A line-level xover (whether passive or active) is a separate component, external to the speaker. It receives the full-range (again, all frequencies) line-level signal from the preamp, separates that signal into two or three parts, and sends each signal to a separate power amp, one amp for each frequency group. Each power amp then amplifies only the frequencies it receives, and powers only the driver(s) it is hooked up to. Line-level xovers can be active, or passive. The xover frequencies and filter slopes can be set---no choices provided, or adjustable.
Here’s an example of the difference between a speaker-level xover and a line-level one: My first bi-ampable speaker was the original Magneplanar Tympani T-1. It came with an internal speaker-level symmetrical 1st-order (6dB/octave) xover at 3200Hz. There were two sets of input jacks on the speaker, with a jumper cable connecting the two. If not bi-amped, the jumper was left in place, and a single power amp connected to one of the set of jacks. Audio Research, the Magneplanar distributor at the time, recommending bi-amping the T-1, and offered a passive line-level xover, the PC-1, with which to do so. The PC-1 created a simple, symmetrical 1st-order filter at 1000Hz, the recommended bi-amp xover frequency. The front panel contained four level knobs, high and low frequencies for two channels. When bi-amping, the jumper connecting the bass and m/t panels was removed (which defeated the speaker-level xover), each speaker jack receiving it’s own amplifier channel output cable.
The ultimate ARC/Tympani T-1 system at that time was with the speaker bi-amped with a D-51 on the m/t panels, and a D-75 on the bass panels, in horizontal bi-amp fashion. I couldn’t quite swing that, so had a pair of D-51’s, which I used in the vertical bi-amp fashion you recommend. Yes, I completely understand your rational for recommending the vertical arrangement, and it has it’s advantages. My dealer (Walter Davies, now known for his Last record care products) suggested it as it allows the left and right channels of the amp to not be in competition with each other on power-demanding bass transients, as they are if used horizontally. That leaves more of the amps power reserves available for the bass channel of each vertical amp. On the other hand, having one channel of an amp seeing low frequencies and it’s other channel seeing the mids and highs, as it does in the vertical arrangement, negates one of the advantages of bi-amping---the freedom from low frequencies inter-modulating with mids and highs. Those are trade-offs, the user free to choose between the two forms of bi-amping.
The op stated he wants a passive, 12dB/octave, line-level, high-pass xover. I have installed a passive (a single capacitor on the input jacks of a power amp) filter to create a 1st order-6dB/octave xover at 100Hz, but I believe it requires an active circuit to achieve 12dB/octave. I could be mistaken, so perhaps someone else knows better.
So sfall, I assume your speaker has two sets of binding posts? And you connect a 2-channel power amp to the speaker, another identical amp to the right, each amp channel feeding one set of speaker binding posts? And both channels of each power amp receive the same full-frequency signal from your pre? Is it not then obvious that your speaker’s internal xover is still doing the filtering---at speaker-level, not a line-level xover doing the filtering? If you want to call that bi-amping go ahead, but that’s not how the term has traditionally been used. You’re right about one thing, though: This type of "bi-amping" absolutely requires the use of identical stereo power amps---the input sensitivity of all power amp channels, and the amp’s gain characteristics, must be identical for the speaker’s internal xover to filter as intended!