thoughts on filtration for the new tank

[JC Pollman]

As I plan my next tank, the wheels are moving (or perhaps grinding) at full speed, and I have tentatively come up with several key parts that I think will make a difference from the last tank. This one will be 84x30x24 and be a plywood tank again. I had a DSB in my last one, and it gave me problems, plus, at the 5 year mark, when I pulled a handful out, it really stank, so I want to avoid that again. But I also think barebottoms have problems too, and in the end, getting the gunk out of the tank is most important to a long term happy aquarium. I am NOT trying to start any flame wars about DBS vs BB, rather, I am trying to come up with a system that uses the best of both. I am thinking of doing the following:
1.Running the return water through pipes in the bottom. I will probably have 4 3/4inch pipes running the length of the tank. The idea is to have a vertical flow of water from the bottom to the overflows. I am planning on using a dart pump on this. The biggest issue I have with this right now is what to put between the pipes. Cement, like the kind used for DIY live rocks, would be ideal, but seems like an awful lot of work. When I "pour" the cement, I will put some plastic tubing in the holes in the pipes to keep the cement out – to be cut off later – and put a layer of sand on top of the cement when it is still wet to give it a “sandy bottom� appearance. The cement will be just high enough to cover the pipes – maybe 2 inches deep. Pond foam might be an alternative, and with enough rocks on top, will probably stay down, but it will never help in the filtration like the cement would. Plus, it needs to be covered with epoxy and sand, and will be a bit soft – particularly with live rock piled on it.
2.I will also have a tunze wave maker to add to the water motion and help get the gunk out. I might add a closed loop as well – not so much for the water motion as, by putting it on a different circuit, I have some backup water movement. And again, the CL returns will be on the bottom.
3.I know this will raise eyes, but I intend to have the overflow water pass through a mechanical filter (probably a sponge of some sort) before it goes into the sump. I want to make it very easy to get at, and will change it out every day. I figure if I have two of these, I can wash and dry one while the other is working, and then switch after feeding each day. I really think this sort of mechanical filtering is essential to keeping the pumps clean and lasting much longer.
4.The sump will have a removable tub with a DBS in it. As time goes on and the sand bed becomes degraded/saturated, I can replace it without disturbing the tank. I am concerned that I can not get a tub which is both of a big enough size to be worth while, and still fit under the tank.

Thoughts?
JC