Spatial Audio (Ambisonic) and calculating sound source position
I know nothing about Ambisonic decoding, but the Wikipedia page suggests:
$$
p(\theta_n) = w(t) + x(t) \cos(\theta_n) + y(t) \sin(\theta_n)
$$
Perhaps you could form that and vary $\theta_n$ to find the direction of highest power?

Note: I realize that's not what the page is really saying. It's saying that this is the signal to feed to a speaker at $\theta_n$... but that seems similar to finding the directionality of the source.  Also note that it does not find the distance of the source, just the $x-y$ plane angle.