Thiel Owners

Unsound, I seriously explored that option with Rob and measured the cut out and increase in volume that would be needed. Concluded that it is doable and the coax driver are available and if you need to replace both the mid-range and tweeter, financially equivalent. I postponed that option and experimented first with a good wavecor mid-range driver and using active dsp tried out various eq and xo designs. As I can compare it to the original 3.5 and 3.7, I am pretty satisfied with my results. 

Tom, I have a pair of sealed subs and if this summer will be slow, I may explore your suggestion more.

Talked several times to Rob about the 10F as the mid-range replacement but never got excited about this option in spite of Rob's claim that it was the best replacement. The difference is size should affect the time coherence even if it may not be audible.

@thielrules, Thank you! What kind of expense would one meet to retrofit the co-axial's to the 3.5's?

slhijb
My pleasure to assist you. Thank You for citing your musical tastes and Rob Gillum's help from CSS.  There are quite a few DIY guys here that are making real progress on finding modern cross-over/driver solutions for their beloved vintage loudspeakers. Take your time persuing this thread, you will find a plethora of information on Thiel Audio 3.5 model.
Happy Listening!

Unsound, the chamber volume for the coax requires almost a doubling of the space that the 3.5 offers for the mid-range driver (using space measurement of the cavity of the 2.7 compared to 3.5). Most practical way would be an expansion of the chamber on the back of the speaker, using a cardboard tube and wood. The front bevel opening would need to be enlarged with an extra inch or so, for which there is enough room. The new coax drivers are about $550 each, not sure about shipping costs. Depending whether you have the tools and skills to do the woodwork, you can estimate your "investment".