The nice thing about good drainage is that every inch deeper you drive the water level is an extra inch in your root zone.
Since you'll probably keep the bed mulched and organic, and/or fertilized, it hardly matters how bad your deep subsoil is at first. Organics and nutrients will leach down into that subsoil, attract soil life, and eventually create good soil.
As long as water drains fast enough to not drown roots, and enough water remains reachable between rains to keep the shallower soil moist.
