My understanding is that you were planning the Dart as the return from the sump with it's outlet in the tank pointing to the Tunze. If that's the case, the Dart is not on a closed loop, it's just a return pump (and with the Tunze lots of turbulent currents)
Closed Loop (CL) implies zero head. The inlet to the pump is directly from the tank and the returns are below the tank water level.
Here's a link to the manifold (with picture)
http://www.wetwebmedia.com/pbh2oret.htm
If you do consider a CL, can do one without OM products but the Waterpix section have lots of pictures of CL:
http://www.oceansmotions.com/forum/v...5afba39b2335f1 (mentioned before I'm bias to CLs, I'm DAS75 with the 4-way and Dart)
Don't have a Tunze and they seem like great pumps but if you want to keep your tank clean of powerheads and since you have such a large tank, might consider the Dart on the CL with a smaller return pump. Really then the only flow required through the sump is enough for the skimmer. Some people have problems with high flow through the sump causing micro-bubbles though I'm okay at about 1000gph with a 75g sump.
Things I enjoyed about starting my new tank was researching through all the different options but then all the choices and opinions can also be a bit frustrating.