The AMB plugins provide certain functionality that are needed to work with ambisonics.  The ambisonic decoder Ambdec will take B-format signals and convert them to speaker feeds, but you need to produce B-format signals! That's where the AMB plugins come in.

My most used plugin (for a 1st order project) is the "AMB order 1,1 mono panner". It takes a mono input signal, and produces a 1st order B-format signal. The azimuth (horizontal) and elevation (vertical) components can be controlled using Ardour's automation.

I presume you've read Jorn Nettingsmeier's papers on doing ambisonics productions? Just in case you haven't this is one of them: "A pop production in ambisonics: Using ambisonics as a production format in ardour" http://lac.linuxaudio.org/2010/papers/6.pdf

-Harry