Petek,
Good work shutting down the fireplace flue.
Other things to check: Any ventilation fans should be checked to make sure they have working automatic dampers. These might be in the kitchen, usually over the stove, or baths. If there is no automatic damper, or the damper is stuck open, it's an open heat leak to the great outdoors.
Check for air leaks between the living space and attic. Bring a caulking gun and a couple of refills with you ;-). Pay particular attention to any modifications to the ceilings - such as new lighting, dropped ceilings, etc. Similarly for the usual weatherproofing around windows/doors/electrical outlets/plumbing on external walls. And of course the obvious set-back microprocessor-controlled thermostat. Lastly, 70F is a bit toasty for Canada in the winter - just my opinion. I keep mine at 65F and wear heavy shirts (thick flannel) or sweaters or sweats at home.
A more efficient boiler would probably help. Since the existing one is keeping the basement very warm, it's probably wasting energy doing that. It might be better to install a register in the basement - that can be controlled - rather than rely upon heat loss, which can't be controlled.
When I bought this California house, the two fireplaces had leaky dampers. Additionally, the stove hood had no damper at all, letting warm air go up the flue all the time. There were many air leaks between the living quarters and the attic. The force air furnace ducting in the crawl space had leaks as well, one quite large. There was zero insulation in the attic.
After addressing all these issues, the natural gas consumption in the winter dropped to half of what it was the previous winter. Plus, the home was far less drafty and much more comfortable. This despite a 1980 furnace in the crawl, which is probably not much better than 75% efficient, and which is now oversize for the better insulated home. One of these days I'll get around to replacing it with a modern unit.