It's interesting to note that a single ambisonic mic (like a TetraMic or a SoundField) can do any of the conventional two-mic techniques that have their mics co-located (positioned at the same spot). That includes, XY, Blumlein, M/S and others.
It can also do lots more, such as three cardioids facing forward and two hypercardioids facing backwards for 5.1, height enabled Blumlein (three figure-8 mics), and many others.
Before recording you would simply place your one mic where the sound is good. You'd choose which mic configuration to use during post-production, back at your workstation. Try XY @ 90 degrees. Don't like it? Try it @ 120. Don't like it? Try Blumlein. And so on.
You can even vary the decode in mid post to highlight a soloist. Or decode the entire recording in different ways for stereo, 5.1, 7.1 10.1 or others. Or decode it using two different mic models, write them out to files and then mix them together later. So you could have a soloist mix, and an ambience mix, and a room mix, and then mix them all together in post.
Add a second ambisonic mic and then you can do any of the spaced arrays (e.g., ORTF).