Hi Tom, glad you are finding my thread useful!
On the 220 -
When I was setting this tank up, I started thinking the same thing that you are thinking. With the DSB, I worried that I would be using up a lot of my tank's height. What I've found though is that the tank still has plenty of depth. I went for an average sand bed depth of 4". With southdown sand, I think 4" is deep enough to develop the DSB. The remaining depth seems very nice IMO. It is just the right depth in my opinion. Any deeper and I would have a lot of trouble reaching the sand surface, and it might also require stronger lighting. I'm very happy now with the 24" depth. If you have room go for a 30" wide tank. That would really be nice for aquascaping, giving you tons of room for rock without having the rock too close to the glass. I'd love to have another 6" width to the tank. That would also make a 220 gallon I think.
What would I do differently -
1) LOTS of things!!! First, I would have 4" to 6" additional height to my stand. This would have made a 55 gallon sump/fuge feasible, and would give more room for plumbing, skimmer height, etc.
2) Outside of the overflow chamber, I would have a 2" bulkhead for the closed loop intake and TWO 1.5" bulkheads for the closed loop output. That way I could use some flow switching device to alternate the closed loop flow between the two branches.
Initially I wanted to do flow switching with the sump return. I found that this causes a lot of problems in my system due to the variation in volume going through the overflow and drains. So I had to give up on my flow switching scheme. If I had one more bulkhead available for the closed loop I could have done it there instead.
3) I would use some acrylic tanks for my sump/fuge system. I am very tight on space under there, and the tubs don't lend themseves to making baffles. I've had some trouble isolating my skimmer and drain lines from the water level variation that the sump sees due to evap.
When my skimmer sees a change in water level, it changes the backpressure on the skimmer and impacts the quality of the skimmate.
When the end of my drain lines see a change in water level, it causes a change in backpressure on the drain line. I have the main drain line "tuned" with a gate valve so that it matches my pump output very closely. This allows the drain to operate fully submerged which is silent and does not make bubbles in the sump. But when the sump water level drops, makes less backpressure on the drain line. That allows slightly more flow down the drain and causes a drop in the water level in the overflow chamber until the flow balances out. It is a self adjusting system but it does need a constant water level at the end of the drain line.
I think if I trim my drain line so that it is just barely at the sump water surface under normal conditions, it will be much less sensitive to this issue. But I have not made time to trim that drain line...
I'm sure there are other things I could think of, but those are some of the major points.
Some things I really like about this setup so far...
- reliable electronic PH meter. Don't buy a used one, unless you get it really cheap and figure in the cost of a new probe. PH probes are sensitive and cannot be left dry, it ruins them. I bought a used PH controller but then had to buy a new probe for it since I couldn't get a steady reading and couldn't get it calibrated. The new probe works great and I love that thing. Calibration is easy too and doesn't need to be done very often. Every time I check it, it is right on the money.
- reliable salinity (conductivity) meter. I have a pinpoint salinity meter which is basically a conductivity meter. It's probe is much less sensitive than a PH probe. This is another great thing to have.
- Reefkeeper. This works as a dual stage temperature controller and digital timer. It takes care of all of my lights as well as two seperate fan channels and my heaters. This is a huge improvement over having a collection of individual timers IME. The newer model has built in PH monitor/controller functions as well as some other nice features. I bought mine used for $175 shipped, saving about $50 over a new one at the time.
- full strength glass overflow chamber. My tank has a glass overflow chamber that is made with glass of the same thickness as the tank walls. Very heavy and very sturdy. It also came with a black acrylic cover to hide the chamber, but I don't use it. I like the glass personally. Especially once it gets covered with coraline. I also like the straight top of the overflow walls. I don't really like the teeth that many overflows use. I have a large 2" diameter x 8" or so tall screen over the sump drain, so I don't have to worry much about it getting clogged. Snails regularly go down into the overflow chamber and keep it nice and clean, then crawl right back out again. Sometimes they even hang out right on the top edge with the water flowing around them.
- the dart pumps. I was surprised at how much heat I get from them, but they are super quiet and really crank out the flow. I have two of them running 24/7 right behind the couch and when the stand doors are closed, I can't hear them running when sitting on the couch. The TV is in this room and the tank has to be very quiet. So far it has been great.
- spaflex - I used spaflex everywhere. There is almost no hard pipes inside my stand.
- I love having the closed loop plumbing hidden in the rocks.
Well that's enough for now...

I could go on and on and on...