... no complete cancellation at 180 degrees. ...
You can only get perfect cancellation with ideal 1-D plane waves, which under the usual idealized conditions, does not attenuate with distance.
In the cases of spherical and cylindrical waves, in free space, the sound pressure amplitudes will be proportional to 1/r and 1/√(r), respectively. What it means is that, at a point in the 180° direction (directly behind), the rear source will be closer to the measurement point than the front source and will have less distance attenuation. If the sources are equal in strength, the cancellation will not be perfect.
Here is an example. The sources S_a and S_b are assumed to be omni-directional point sources in free space and there is no obstacle impeding the sound waves. The source are separated by 0.15 m. The are 2 receiver positions, R_1 is 1 m from the center point and R_2 is 2 m from the center point.
Considering R_1 first. If the sources have equal strengths, pressure from to source S_a is 1/1.075 and pressure from source S_b is 1/0.925. The residual pressure at R_1 will be 1/0.925 - 1/1.025 = 0.15. For perfect cancellation, the strength of S_b will need to be 0.925/1.075 = 0.86 that of S_a.
If we compare this residual pressure to that of a reference single point source of strength 1 (which will result in a pressure = 1 at R_1), the residual pressure will be equal to an attenuation to 0.15 = -16 dB.
Now consider R_2. Pressure from S_a will be 1/2.075 and pressure from S_b will be 1/1.925. To have perfect cancellation, S_b will need to be 1.925/2.075 = 0.93. This is a different number from the R_1 case, which means we can only EQ to provide perfect cancellation at 1 point in the 180° direction line.
In real life where the sources aren't omni-directional point sources and the speaker cabinet is a significant obstacle, we should expect significantly less cancellation (even when in anechoic conditions).