I'm certainly no expert on this stuff, but I can give you some data points for comparison.
Our house is also 1400 sq ft. We are in the warmer area of zone 6, just a little colder than you in zone 5. Our house is a 170 year old farm house with many improvements. In 2011,when we put the geo system in, the house wasn't as tight as it is today, but we are probably not as well insulated and air sealed as your place is.
Some specs for our system design were:
71 Mbtu heating
11.5 Mbtu cooling
10.6 Mbtu water heating
winter design 2 degrees
heating setpoint 70 degrees
2000 ft of loop buried 7 ft average depth
3 ton Climatemaster tranquility 27 with desuperheater
Our monthly average electric usage since the geo system was installed has been around 1050 kwh. The highest month being Jan at 1600-1800 Kwh and the low months in the summer 750-850 Kwh. We have 2, and over the last year, 3 in the house. We are not using any other heat source with the exception of about 4 fires I made in our wood stove last winter rather than use aux heat those nights. We like our house cooler than you do, and set our tstat to 67 without setbacks in the winter. We air condition down to 71 when we use it, but it doesn't get a lot of use.
I would continue to search out energy hogs at your place. Don't forget things like the well pump. You can put a current probe on the legs out to it in your box and rule out leakage to ground.
If you find nothing, I would suspect your heating system might be oversized and you are just paying a little more to run the system than you would if it were sized smaller, but this is just a guess. The fact that the desuperheater doesn't have it's own tank is likely the reason you didn't notice any savings with it on in the winter. Below is a link explaining why a separate tank is needed...see post #5.
http://www.geoexchange.org/forum/threads/desuperheater-two-tanks-one-tank-demand-dwh.396/#post-2838
If you have any geo questions, the forum I linked to above is a great place to ask them. The guys there could probably help you tweak your system, if necessary, to get the most out of it.
You could also have an energy audit done on the house, with a blower door test, to make sure there are no surprises with the building envelope.
edit....I just reread your post above and noticed you mentioned a 765 sq ft addition. Was this part of the original heat load calcs ? If so, you are definitely better insulated than we are. If not, then your electical usage is probably right on par with what we use, especially considering your tstat set point.