I think you’re making the right decision in not going with the ESL63s in this case.
Which coaxial drivers did you have in mind?
And which software have you been using for modelling the crossovers?
No I don't want to use actual coaxial drivers. I'm aware of the errors of the concentric models for sale except for the way Geithain does it (but that probably still has some difraction problems? And at least great restrictions on the tweeter used. And I'm not going to attempt such a thing with my zero experience.)
I want to make a design which in practice behaves mostly like a dual concentric design by making the distance between the acoustic centers of the mid and tweeter as small as possible while still mounting the tweeter on a baffle above the mid, combined with a low crossover frequency this will give near dual concentric/coaxial radiation pattern in the vertical plain.
Here a picture of a Butterworth crossover radiation pattern:
And here a picture of a Linkwitz-Riley crossover radiation pattern:
The text that comes with the pictures states that these are the radiation patterns for a 1700Hz crossover with the drivers 7inches apart (haven't calculated if this is correct but it seems about right).
Now of course in reality the situation is often even worse where a crossover between mid and treble is most often 3000 to 4000Hz giving more lobes / cancellation axises closer in angle to the on-axis.
And I belief most often the Linkwitz-Riley is chosen for the mid to bass crossover or bass to sub crossover and the Butterworth one for mid to treble crossover. (because all angles together/room reflections/reverb add up to flat for the Butterworth all pass and the Linkwitz-Riley doesn't add up to flat when you combine all angles but adds up with a dip of -3dB(I think?) except when the crossover is very low and the drivers close enough that the cancellation axis is more that 90 degrees from the on-axis.
And just look how finicky, even in the above in reality good case scenario, the Butterworth on-axis is, if you move just 5 degrees lower you already get an almost 2dB peak (and frequency ripple all around the crossover frequency, very audible as I tested with a simulation with headphones) and 5 degrees to high you get an even larger dip already.
The Linkwitz-Riley crossover has a more stable patch around the on-axis, you'd have to move 10 degrees or so up or down for an about 2db dip in the above example (and again, in many real world situations much less movement would be needed).
Now of course in a very live room with many diffuse reflections this evens out mostly. But I will be listening near-field in an almost completely dead room. And again testing this with my headphones and simulated off axis crossover sound I can tell this is all incredibly audible and totally destructive for soundquality / transpancy etc. This is a very big part of what makes speakers sound bad I can hear and recognize from the sound of it.
So my solution will be to use the Linkwitz-Riley crossover (perhaps 24dB/oct perhaps 36dB/oct) and place the mid and treble drivers so close together and put the crossover so low that the first cancellation axis up and down is more than 90 degrees from the on-axis so in effect the cancellation axis is never fully there anymore (virtually behind the speaker) and the on-axis response is within half a dB or so over a wide verticle angle.
The only downside to this is the low crossover point which needs to be at least below 2kHz (preferably 1.5kHz or so) depending on how close the mid and treble drivers are (10cm is about the max depending on crossover freq but even closer seems very doable).
I don't understand why most speaker builders don't already do this. That vertical phasing around the crossover is really really bad sounding and I now realize/ recognize it has been bugging me pretty much my whole life. Feel good about building a speaker that doesn't have this.
For amp I think I'll indeed go with the Hypex FusionAmp, probably the FA123. I finished reading the manual and filter designer doc. It's great what it can do! Only no fir filters (yet, seems like this may come later with an update there's already a slot for it in the filter designer) but I wasn't planning on using those anyhow.
Still looking at drivers and several other things, will probably be a few weeks before I actually begin building it. Will post here once I've built them
(so far only built a sperical speaker with a full range driver, which was a success though, learned a few things about woodworking and damping box resonances with leadbitumen, bracing and stuffing etc. So this is a step up but not that much different if I keep it sealed, port may be different story)
Edit: And a big thanks to amirm for saving this post!
(had accidentally deleted it when trying to delete a duplicate attachment)