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Beta 15 has 2" voice coil (weak for 15" cone) and Q .58 which is good for passive dipole when EQ is not available, but gives up too much cone control and tonal detail when active EQ is available to boost the bass. High Q allows more low bass but has less detail. Kappa 15A has 3" voice coil, and Q .32. It will have much better cone control and power rating, and better transient response. If there is too little bass you can easily boost the bass electronically with your active crossover. Beta15 is poorly damped which will blur the detail at all frequencies, but more noticeable in comparison to 1808 as you move higher into midrange band, so you would want to roll it off as steeply as possible and as low as possible to avoid smearing the midrange from the very detailed 1808. Delta 15 is much better damped electrically and mechanically, so it will blend with the detailed 1808 up into the midrange so you can run it a little higher to get better integration. In either case you must adjust the crossover freq by ear, because it affects the meatiness and fullness of the tone through the low midrange and upper bass. Especially if the two drivers are very different tonally, like 1808 and Beta. If drivers have more similar tonal color then it is less critical. XO should be steep, probably 24dB/octave, and probably 150-250Hz.My crossover freq with 2x12" bass drivers Q.7 and one 5" full range driver was 175Hz. If you like a fuller sound, richer bass then XO freq might be higher so woofers play higher. If you like clean detailed bass then lower so 1808 plays lower. But 1808 being smaller cone doesn't grip the air at lower freqs, so it will slip and lose detail and it can't play loud and low. So the louder bass you need the higher the XO must be. Voicing a speaker involves many compromises.Above 150Hz, if possible. As low as 100Hz if necessary. Below that it will blur bass detail (in comparison to dual 15" cones) due to low acoustic impedance. The lower it is forced to play the more midrange distortion you get from the low freq cone movement. So getting out of the bass range is ideal. But the bass drivers must be able to blend with it through the XO band and your woofer candidates will have trouble playing cleanly up past 300Hz. Beta won't play super clean at any freq, but it will play low and loud with blur without any external EQ boost. Trust your ears. The speaker system is intended to serve your brain, through your ears. It is not intended to serve your ego obsession with scientific validation through measurement. Make small changes slowly. Listen to what each change does so you can learn what is happening, not just reacting emotionally to instant AB comparisons. Allow time for the changes to settle in, and your brain to adjust and assimmilate the changes. At first, yesm, the changes will be fast and large until to get a basic workable system. Then listen a lot between changes. Take notes about what sounds good and what sounds bad across lots of different music. Don't use the same test tracks. Allow your intuition and imagination to assist the tuning process, allow new music to come in.Have a reference speaker that you like, which makes most music sound pretty good to you. It may be your car, or other hifi system, or home theater, whatever. Compare it to your dipole speaker voicing.Either will work, but you have to try each to see which sounds better tonally.Beta and Kapa have higher sensitivity than 1808, so you can wire them in series for net 16 ohm load and retain the specified sensitivity rating for one speaker. In other words, 2 Kappas in series is 100dB sensitivity at 16 ohms. This would be good if you were using tube amps on the bass drivers. But you might find that your ampliifer prefers and sounds better playing at higher current, so a parallel load would be better. 2 Kappas in parallel is 4 ohm nominal load, with 106dB sensitivity, so you would have to attenuate the woofer signal in your miniDSP mix to match the SPL level of your 1808. Easy to do.Try both. With Kappas in series the tubes might be nice in the bass, especially if the Hafler sounds better than tubes on the 1808. The different amplifiers will have different voltage gain which adds another variable to compensate in your miniDSP level mix.You'll want to generate a few different profiles in DSP so you can swap gears around for comparison without the delay of manually adjusting all the gains. If you like to play very loudly, and like bass slam on popular genre music then you might prefer the superior current sourcing advantage of SS amp on the bass drivers. Try both and see. Give each adequate time to listen to assortment of different music to learn the pros and cons of each. Don't just play Led Zep and decide that's it! Let you brain adjust to both so the pros and cons are obvious and clear. Then you can decide and make fwd progress.A successful speaker project is half building it and half tuning it. It takes time. The real work is mental, in design stage and in voicing it. Allow 6 months before you feel comfortable to stop messing around. But it should sound pretty close after 1 month of diligent listening with minor constructive tweaking with firm hand on the tiller according to your notes.Go ahead and measure if you want, it can speed up the process dramatically especially in the beginning, setting up XO and levels. But don't let measurements make final artistic decisions. You are not professional mastering engineer, it doesn't have to be flat. Make it sound good and feel good or else you'll never use it. Get the Kappas or even a different brand of woofer to maximize the transient response in the bass band. Choosing active XO/EQ enables a higher level of sound quality with dipole bass, so don't piss that away on high Q woofers which the active XO cannot control. EQ can easily raise Q (force woofer to play lower and louder, but EQ cannot lower Q (force it to play more detail and bass slam.) Any "full range driver" will beam the treble frequencies. This is why tweeters are small, to make them spray the highs more widely. 1808 uses a wizzer cone to simulate a smaller tweeter, but it still beams very strongly. You don't want to be in that beam. Your ending FR will be rolled off. That's OK. Musical treble goes up to 8-10kHz. Audiophile fireworks treble goes higher, but you're not building an audiophile hifi speaker by design. If you listen outside the beam, them EQ to flat 20kHz then the reflected treble will be too hot. Your best sounding compromise will be measured rolled off treble. Trust your ears! Since the 1808 requires listening off axis, you can toe it to the side or you can tilt it back to aim the axis away from you. Tilting it back has a couple advantages. The beam lights up the ceiling more, so you get more height image. The beam is always shot over all the listeners who are seated at the target listening area. Tilting baffle back aligns the 1808 acoustic center with the woofers' acoustic center for flatter phase less blurring through the crossover zone. Meaning the sound from 1808 and sound from woofers arrive at your ear at the same time, so they don't blur each other by slight delay caused by different travel distance. At these lower frequencies in the bass crossover where they are both playing same notes together, a couple inches of phase distortion is not a big deal, especially with a slow woofer like beta, but with kappa it will be more audible so just depends on the results you're shooting for and how detailed you want to make your design. An alternative for dipole bass that is popular with AudioCircle members is to use servo subs offered by GR Research. They are high Q driver that allows playing low and loud with low power, but the Q is electronically lowered by a feedback sensor on the voice coil. So the driver cone is forced by the special servo amplifier in feedback to follow the (EQ'd) input signal accurately. It is functionally the same as using a well behaved, self-damping driver with low Q, and boosting the amplifier input signal to play low and loud with EQ. If you already have the amp DSP and drivers then stay the course. MarvintheMartian's advice about the sound quality of inexpensive electronic crossover appliances is worth considering. The cheap digital XOs are tempting, and probably worthwhile for your first step, but if you are already "ruined" by high end sound quality then you will want to move on to something better. Passive XO is an option, but then your forced into high Q woofer. Active analog XO filters can have excellent high end sound quality, especially if you know the quirks of the buffer amplifiers you are using. You can use the miniDSP to determine what functions you ant to apply to your input signal, then you can design analog filters to perform those functions to get the high end sound quality. Or you can buy a digital XO that has high end sound quality like DEQX. Or you can enjoy miniDSP and not perceive any deficiency. Give it a try, might be plenty good enough for 1808 even if it's not good enough for a audiophile dome tweeter.
