Those magnum's are nice. I've seen one run and it was great for radiant heat... but I know the blowers on it wouldn't be enough to heat an entire house. It isn't a furnace. Its blower might be 400 cfm if your lucky, where a furnace blower will probably be closer to 1000 cfm.
You are pushing hot air, however at a very low volume. You should put in another register and get a booster fan for each one. I think your mag has a 6 inch take off.
It might not make since… but this is how I would try it. 6 inch from mag going to a 6 inch Y, then have two 6 to 8 increases. Have those boosters on low/med/high fan speed controls (not light dimmers). Then the 8 inch ducts go to your registers.
YYYYYYYY |||||| 8 in BOOSTER||||||
Y 8 In Y |||||| 8 in BOOSTER||||||
YYYYYYYY |||||| 8 in BOOSTER||||||
-------6 in from mag---YYY
-----------------------YYY
YYYYYYYY |||||| 8 in BOOSTER||||||
Y 8 In Y |||||| 8 in BOOSTER||||||
YYYYYYYY |||||| 8 in BOOSTER||||||
I'm heating my house nice with boosters tied into my existing ductwork. I'm not sure what my registers temps are but I have a temperature probe about 4-5 feet from my duct fans my air needs to be around 120 degrees to keep it steady at about 10 degrees. -1 and windy it needs to be 140 degrees. That’s with two 420 8inch duct fans, 840 cfm. Those temperatures go up fast when I turn them off when I shake it down. I get 160 degree temps by the time I turn my fans back on, my stove has a small fan on the back that says on the whole time.
http://www.airboosterfans.com/ is a great place for duct boosters.
http://www.warmair.com/html/ducts.htm Here is a little info about duct work and placement and register size. One simple point is to put your registers near outside walls. I try to put new registers by outside walls next to windows. The draft a window naturally have might actually help your air flow. Your locally library has books on central heating, and heating with solid fuels.