Not a one size fits all question, but all other things being equal(depth etc) Is good topsoil more important than what's underneath, or vice versa?  Looking at pictures of how roots are supposed to grow, the majority of their depth and biomass is in the subsoil.  But they obviously need to get their healthy start in the topsoil.

Around this farm, most topsoil looks poor compared to mellow black, ours is grey clay, very low OM, only 8 to 10 inches deep, compacts really badly, shrinks and cracks, dries into unbreakable clumps the size of a basketball...  The subsoil is also clay, and holds moisture extremely well, and on a dry year like this, we grew crops way beyond what should have been possible given the top soil and lack of moisture.  I've always thought that annual plants can't penetrate our subsoil, and must rely on moisture to wick up from below, but need to do some more digging of roots this year and reevaluate that concept.  I did some deep ripping/subsoiling last fall, and the results "look" very favorable.  And ironically, my logic was to get rid of excess water.  Not far away, there is lots of better looking black topsoil, but sits on sand or gravel, and doesn't do as well on stressful years like this.

So with greenhouse conditions, could you grow an excellent crop without topsoil?  With good enough topsoil does the subsoil not matter?