For depth, how bad is your existing soil?
Once you have gardened in one spot for a few years, the organic mater, water, fertilizer and roots in the RB soil will tend to soften, break up and enrich the original soil under the added soil.
After a few years you may be able to step inside your raised bed and dig down 12 or 18 inches deeper than you could when first creating the bed, and amend that subsoil to the point where potatoes can grow into it.
Especially if you grow a few crops like daikon radishes that seem to have jackhammers in their root tips. Or a catch crop of alfalfa or maybe fall rye or clover.
When I make an RB, I like to do some pick work on the subsoil and mix in something organic and chunky like bark mulch, or just organic like manure. I figure that will give roots and worms a head start on improving the clay.
