In my experience, a higher action will give the player a richer tone and - usually - a louder instrument, as well. Many bluegrass guitarists keep their actions fairly high for just this reason.
As for lights versus mediums, the only way to produce tone on an acoustic guitar is mechanically, with the vibration of the top being driven by the vibration of the strings. The more tension that the strings generate, the more the top will vibrate (up to a certain point.) Medium gauge strings generate more tension than lights, so that has its effect on the tone.
So it isn’t surprising that changing the string gauge and action had an audible impact on the tone of your guitar; it would have been more surprising if it hadn’t.
Everything is a trade-off with an acoustic stringed instrument. Change one or two elements, and that will manifest itself in the sound (and perhaps playability) one way or another.
Hope that makes sense.
Wade Hampton Miller
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