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Both 2x12's are done and ready for driver install and wiring.

Installing the drivers and wiring.

Drivers installed and wiring for the speakers is done. Now for some testing.

The first servo sub is working and trying out the test setup with a ground plane measurement. Will do a nearfield measurement to see if things are working as they should.

The nearfield measurement of the upper woofer looks good, so off to building the second 2x12.

The second 2x12 H-Frame is loaded with servo drivers. Now starts the last wiring. Both HX300 amps are working.

Both subs are now running and measurements looks good. Now to experiment with the digital crossover for the OB planar head.

What is the reason for reversing one woofer. I assume they are firing in the same direction

Yes, both are firing the same direction.

From Sigfriend Linkwitz it helps lower harmonic distortion, if I am understanding the information. His W-Frames use the same concept.

If you look at the photo you will see that one driver is pointed in the opposite direction from the other. If they were wired in phase one would cancel the other as the cones would be moving in opposite directions. By reversing the polarity of one in relation to the other the cones will move in the same direction at the same time. This does tend to reduce 2nd order harmonic distortion although I am not clear on the mechanism why that happens. In reality, the most important thing to do for distortion reduction is to keep the driver within XMax limits.