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This really sounds like a crossover issue; here's something to consider

Posted by Ralph on May 12, 2023 at 10:47:27:
If you don't crossover the woofer its impedance will be in parallel with that of the midrange 'full range' driver.

OTLs generally don't like lower impedances. If you have 8 Ohm drivers, the speaker will be nominally 4 Ohms thru the 'full range' driver region.

I know 'full range' driver guys don't like crossovers. But the simple truth is 'full range' drivers deserve the quotes because there really isn't any such thing (else you'd not be trying to use a woofer). If the woofer is active at higher frequencies, you'll get combing effects as the woofer is alternately in phase and out of phase with the other driver. The ear interprets this sort of thing as harshness.

So use a crossover. Getting the bass off of the 'full range' drive is a good idea, since that reduces Doppler Effect distortion.

The crossover will help linearize the impedance curve, which will make for an easier load for the amplifier. A ZOBEL network really has nothing to do with it!