Aud1073cH

This is cool. Would you mind sharing the DWG and/or DXF file with the community?

With any new or experimental design, it is good to prototype first. You might connect your preamp/ transformer/ mic with alligator clips to experiment and see what results you get. Your preamp should see the output impedance of your transformer. With my design, I knew I was going to get rid of all the high frequency, so it didn't matter so much. Although this is an interesting idea. I may need to try it sometime. Try a few things, and go with what works. Don't be afraid of failure.

The best photo I have of the connections is the one posted on Step 7., Much of the complicated design was experimental, and rather unnecessary. I built a second one later with a single coil, and without the crazy coil selector or 1/4" jacks. The original Yamaha model had no extra switches or knobs (photo attached).

It is actually much simpler to just go straight to the XLR like yamaha did (photo). or just connect to a 1/4" instrument cable and plug into a DI box.

You can go directly from the speaker to the XLR connector - that's what Yamaha did. The DI transformer will just help with getting closer to the proper impedance for a mic line.