thoughts on crossover, with these drivers?
I have seen the midBass, but am not at all familiar with the FR.
At the frequencies involved going active makes things way easier… and depending on how you do it, could even be cheaper. The size of the passive parts is significant, and good ones are not cheap — ie in our A12pw/A7.3 MTM XO @ about 250 Hz, there are a couple 240 uF caps and some equally big chokes. Even the 450 Hz passive on Tysen V2 has a 240 uF cap.
We design to be able to use 1st order PLLXOs which have minimal parts cost and no electronic haze. Some HT receivers have built in XOs that use channels 6&7 to be able to bi-amp. Something like miniDSP has huge flexibility, but the analog sections in the cheapest ones may not be up to your standards and the best ones avoid ADC at the front end (but sort of restrict you to digital sources). Often people will use something like miniDSP to work out whta filter is best and then try to implement that pssively or in an anaog line-level XO (ie the modified FirstWatt B5 Nelson is working up for the Linkwitz LX-mini).
A PLLXO can be 2 Cs and an R, and if you want to minimize one of those Cs can be the input cap on the HF amp. A series R & shunt cap are the LP filter (one assummes when Danny says no series R in the LP he is talking about a passive XO). At least 1 amp (usually the LF amp) needs a gain control to set levels.
A passive XO is much harder, as one has to deal with the variable impedances of the drivers particularility down around their resonance frequency where the XO is being placed.
When choosing the XO frequency one has to take into account driver centre-to-centre and baffle step F3. Ideally one targets the XO at less than a quarter wavelength of the centre to centre, and within the range BS (F3) times (the range 0.707 to 1). No BS is then needed on the midTweeter.
As the XO approaches 200 Hz the greater is the need to go active.
Do you have a 2nd amp already?