OK read both articles and after reading I am struck by the following line in Julian Sprungs article:
"The gravel in Jaubert's systems[using a plenum] has been termed "live sand," as it has in other natural systems such as Eng's and Dr. Adey's. The use of this term in connection with Jaubert's systems is misleading because the size of the grains used should be much coarser than sand. "
So essentially a plenum works well with non-sand substrates. Meaning you should not have a DSB and a plenum. Here is another line about the use of sand or layering of sand over gravel:
"Since the sand bed does not function as a mechanical filter, and because it must allow good diffusion of water in order to function properly, the idea about layering makes no sense. "
This means with the plenum you would have the over-oxygenated water stuck below the sand and not diffusing at a helpful rate upwards.
If your boyfriend insists on keeping the plenum I'd point out that line in the Julian Sprung article.
Lastly - if he is trying to point to a plenum as easier to maintain I would point to this line from the Sprung article:
"Some aquarists have reported a problem with gravel fusing into a solid mass of rock. This occurrence of course can to some degree prevent the proper functioning of the gravel bed as a biological filter, though if the mass is still porous it might not be so problematic. "
The last two points I would make in defense of a DSB over a Plenum would be from the Deep Sand Bed Article by Dr. Ronald Shimek
1st: "Siphoning, disturbing or cleaning of the sediments will result in significant removal or mortality of sediment organisms and may severely damage the functional aspect of the sand bed."
Meaning - you do not have to worry about the fusing that goes on with the plenum sand bed and
2nd: "Have you ever had a problem with hydrogen sufate (sulfide? Im not a chemist) and if so, what he does about it?
Only when I tried a plenum. Bad news, there. In my sand beds, never have had a problem."
One less chemical you need to worry about popping up in your tank.
I know several other people can chime in either way - but given the articles and the fact I am trying to blow off 20 minutes of actual work time ... I decided to go with what was on hand .. so to speak.
Good Luck
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