Hi Andrew,
Thank you for your interest in IRD audio components.
Here are the answers to your questions: ( a bit windy...)
1) Active crossover filters (AXOs) can be used more than one way:
1. You can integrate a subwoofer (1 or 2 subs) into a 2-way loudspeaker system using a 2-way AXO with the XO point (a 2-way crossover has only one XO point) set to 40-100Hz (depending on the main loudspeakers bottom end).
The AXO lowpass (LP) outputs drive the amps that are connected directly to the subwoofer drivers (this is the most efficient method of driving subwoofer(s) ) and the highpass (HP) outputs drive the amps driving the 2-way passive crossover loudspeaker system (no work is required on the loudspeakers for this application).
Using this method you can get a very good, possibly seamless integration between subwoofers and a 2-way (bass shy) passive crossover loudspeaker system.
2. You also can use a 2-way or 3-way AXO to directly replace your loudspeakers passive crossovers, selecting the same XO points as the original loudspeakers passive crossover used.
You simply re-wire the internal loudspeaker binding post connections directly to the drivers (you need a separate set of binding posts for each driver) and leave the disconnected passive crossover intact inside the loudspeaker in case you ever want to switch back, like if you sell the speakers. Each AXO output drives an amp that drives the related speaker driver directly.
The benefit of this method is that all the drivers are driven in phase (with much better control than a passive filter can offer) which can eliminate the frequency sensitive radiation pattern changes that are associated with many passive crossover networks, plus, the speaker efficiency is increased by as much as +3dB do to the removal of the lossy passive filters. This means you only need half the amplifier power to reach the same SPL as with the original passive crossovers.
3.Another way to use AXOs is to disconnect the shorting bars on your loudspeakers, that bars that tie the LP & HP passive crossovers together allowing you to use a single speaker wire cable. Now you can bi-wire, bi-amp the passive crossover loudspeakers, or use an AXO and bi-amp.
Using a 2-way AXO (for 2-way loudspeakers), the lowpass (LP) outputs drive the amps that are connected the bass drivers passive crossover filters and the highpass (HP) outputs drive the amps connected to the tweeter drivers passive crossover filters (no work is required on the loudspeakers in this application). The AXO separates the frequencies at line level and the loudspeakers internal crossover has little if any work to perform. You will need four amplifiers and four sets of speaker wires.
This method allows you to use smaller amplifiers and obtain the same loudness levels as a larger amp would give, this is because of reduced clipping level distortion (because the mids and high frequencies are not ridding on the bass wave waiting to be clipped during bass heavy passages).
2) Yes, you can simply use the Purist and four MB-100s to power your bi-wired Triangle loudspeakers and this should work just fine with the efficient Triangle loudspeakers but, it may sound just as good using only two MB-100s since the Triangles are so efficient. IMO not much is gained bi-amping bi-wired high-efficiency loudspeakers. Now low efficiency loudspeakers are a different story.
My first thoughts are: In general Triangle loudspeakers can be quite good stock (I'm not familiar with the exact model you are using) and they are also high-efficiency, so I would not recommend taking out the original passive crossover networks (right away anyway). You might want to try a Purist preamplifier and a pair of MB-100s, this might be all you really need. You can always add more later.
