>> Dig out the ground underneath the raised bed to about a spade depth, and then you will have about 18 inches in which to grow the plants.
I strongly agree (as long as your drainage is good enough to keep the below-grade part from flooding during a heavy rain). I have to dig the 'floor" with a little slope towards a slit trench for drainage.
Giving a raised bed a below-grade foundation of loosened or improved soil deepens the roots zone and encourages the deeper soil to become more organic and better soil.
As compost in the top 18" breaks down, I think and that some of the 'drippings' soak into the subsoil and attract worms. Over time, roots will also dig down and gradually improve the sub-soil to give you an even deeper root zone.
I like to mix medium and large bark chunks into the under-layer, on the theory that they will add some drainage and water retention, but mainly because they will break down slowly, like over three or more years. They release a little organic matter, and also create small voids or air spaces deep under the bed as they decompose. I hope that helps the deeper soil levels to drain and stay aerated.
As clay and silt settle out of the the top 18", these voids give them a place to emigrate to, without plugging and clogging the soil. And if the soil above the voids settles or subsides into spots where bark nuggets used to be, I imagine that that movement creates mini-voids throughout the soil column, like ongoing deep turning with really tiny spades.
Speculation or wishful thinking? Either way, I don't think it hurts to mix some long-lasting soil amendments into the layers beneath a raised bed.
