If I had the bucks, like won the lottery and had unlimited funds and free time - I think it would be interesting to run several tanks with near identical bioloads/fish/inverts, and near identical feeding habits.
Each tank having a different biofiltration method.
One tank - DSB only
One tank - live rock only
One tank - DSB and live rock
One tank - shallow sand bed
One tank - refugium only
One tank - protein skimming
I think you see what I'm stating here.
Many many different ways to run a tank - and many variables that come into play regarding exporting excess nutrients and natural forms of nitrate control.
If I could do this ( am I'm sure others that do research have done just that ) I'm sure I could resolve some of these questions in my own mind for myself.
I've run tanks with wet/dry's, with live rock, with CC substrates, with DSB's, with and without protein skimming, some with high water flows, some with what I would consider medium to low water circulation ... and for the most part, all of them did okay.
But each had different loads, feeding levels, cleanup crews, water change routines and each had different nitrate readings over time.
So many of the variables changed - and I had no "control" tank that stayed a relative constant.
Hard to tell what contributes to a lowering of nitrates unless you keep a control tank, OR the ONLY thing ( repeat ONLY thing ) you change is using a DSB - and keeping EVERYTHING ELSE constant.
And I continued to make changes to the tanks as I went along learning and try new techniques.
My point is that for me it was an accumulation of learning, and moving towards a DSB with live rock, putting in a good size algae refugium, learning how to maintain all my water parameters and keeping a lower more balanced bioload - AND a HUGE REDUCTION in my feeding routine - that I believe ALL total contributed to my less than 5ppm nitrate readings I get now. Still trying to get to that zero reading for any extended period of time - just haven't got it figured out yet I suppose.
My tank keeping methods "evolved" as I learned more and read more about nitrate control.
Plus - tanks MATURE and normally become more stable as time goes on.
Taking readings from my 75 a month after setting it up vs. 6 months or a year or more later - I would expect different test results due to the age of the tank.
Presently - in my 75 reef tank - I keep the nitrates down around 5ppm with a 6 inch DSB, 75-100 lbs of live rock, harvesting macro algae from a healthy refugium that also has a 6 inch DSB, extremely light feedings, using RO/DI water for all top offs and saltwater mixing, and doing about 15 gallon water changes every 6 weeks.
I'm pretty sure I could get the nitrates back up if I really overfed this tank each day.
This is just one variable that I could easily change, and would make all the rest of my biofiltration a "moot" point.
Lots of variables - lots of ways to run a tank.
But .... I am still a huge fan and supporter of having a functioning, healthy, deep living sandbed. It along with all the other things I've learned and put into practice ALL contribute to lowering of my tankwater's nitrates.
Not just one thing by itself.
It would be interesting to know how the amount of live rock vs. live sand compares in the wild <img src="graemlins//yeahright.gif" border="0" alt="[yeahright]" /> Just food for thought 
