@mattstat .. xover in the driver, thats not what I wanted to hear. If that is the case then I guess its not too much of an issue, I can build another xover. I have all required software and measurement tools for it.
@planet10 your recommendations, are they coaxial or full range drivers? Whats an FR please?
@planet10 your recommendations, are they coaxial or full range drivers? Whats an FR please?
I'm not trying to discourage your experiment, just bringing up some details that might or might not be relevant to you.
With a typical passive crossover and co-ax you may not be able to maintain the time/phase coherence of the original Thiel design. Modern coaxial drivers tend to have at least a minor Z offset between the midrange and tweeter acoustic centers. It's annoying, but it's there in many.
Finding one that that would let you use 6 dB/octave crossovers is a little tougher as well.
Sensitivity of typical small co-ax's can also be an issue if you want everything passive. The Thiels were pretty high at about 90.5 dB.
If you're going to go active/DSP, all of this can be corrected.
At any rate, here are a few co-ax's that are about the right size (but most have lower sensitivity in the midrange than your speaker):
SB Acoustics SB12PFCR25-4-COAX 4"
https://www.madisoundspeakerstore.com/coaxial/sb-acoustics-sb12pfcr25-4-coax-4-coaxial-4-ohms-round/
SB Acoustics SB12PACR25-4-COAX 4"
https://www.madisoundspeakerstore.com/coaxial/sb-acoustics-sb12pacr25-4-coax-4-round/
Morel Integra 424 - this one claims time alignment, but I have not worked with it so I'm not sure. Parts Express' page says "near perfect time-alignment," and I'm not sure what that means.
https://www.parts-express.com/Morel...-Source-Coaxial-Full-Range-297-024?quantity=1
Sica 4 C 1,5 CP - highest sensitivity I've seen in a quick search.
https://sica.it/prodotto/4-c-1-5-cp/
Why do you want to substitute the original mid/tweeter coax?
Actually, it is a small fullrange driver with what can be described as having an internal "mechanical" crossover - somewhat similar to dual cone fullrange drivers.
The easiest way for substitution is to find good conventional fullrange unit (not dual-cone!).