It is recommended that you use a comfortable position, so the armchair is probably a good idea.
You do bring a good question to which I don't know the answer to. Maybe my experience might help in figuring it out.
I've started meditation before coming to ayp with conventional cross-legged on a pillow and self-made wooden prop (not a good posture in any way) which naturally transitioned to burmese posture, with a sock, on a simple folded towel (legs parallel each other) though it isn't entirely good on the ankles and can't transition to quarter lotus without putting strain on lower back, to which a zazen pillow (zafu) might be the answer.
As much as I worry that the posture might be interfering with meditation and my energy levels due to not being entirely balanced and relaxed, it just feels very good. I don't know the true meaning of grounding or overloading (though in the past I did get persistent head pressure from standing meditation) even though I've done grounding practices in the past, yet there is something about sitting that makes all that worth it, part of which no doubt is the reward of a very gradual transition to becoming more stable and comfortable.
"Pranayama and meditation interact with the effects of siddhasana to greatly expand the flow of prana and pure bliss consciousness" -
Lesson 75.
oh, and welcome to the forums, shibumi!