Careful here King. A DSB is still an effective way of additional filtering capacity for a reef tank especially in much larger systems. There is an understanding of the system that requires a lot of education on the part of the reefer that is looking into to it though. Here is a little blurb that pretty much cuts to the heart of maintaining a DSB. The qualifying word here being "maintaining".
"The DSB in the Live Sand Filter depends upon "sand stirrers" moving through the sand, keeping pockets of toxic gasses (produced by decaying organic matter) from forming. Invertebrates, such as Sand Sifting Sea Stars, Sea Cucumbers and Snails (the Nassarius is legendary) plow through the substrate, consuming detritus and uneaten food as well as releasing the toxic gases in small quantities, before they can grow to dangerous levels. Wrasses, such as the Yellowtail Coris and the Dragon Wrasse, which bury themselves in the sand when frightened or seeking refuge for sleep, also do a great job of stirring the sand, however they do not consume detritus, as other stirrers do."
If you note in the above quote there is a need for the stirrers in the system to keep it viable. This cannot be done by the reefer stirring the sand bed. If you note some of the stirrers listed they are critters that would decimate a small tanks sand bed of the flora and fauna required and render it useless. Hence the need for a large system to make the DSB a viable option.
It appears that the OP has a 55 gallon tank from their profile. I would agree here that a DSB in this tank would not be advantageous and would go with your recommendation of substrate depth depending on the critters they may want to keep. For instance if a jaw fish is in the works I would increase your recommendation to 2-3".