I love New Hampshire! Spent many summer and winter weeks there as a kid. Winter losses can be that bad here too, and worse. Of my four colonies last season only one survived - but as I mentioned, there was increase! And now I have four colonies again.
If you’re certain your frames had swarm cells on the bottoms, you might not prevent a swarm from issuing by putting on a second brood box. It might be more effective to split your nuc, if you can believe it. Best to do some reading and figure out what matches up with your situation. I’m going to look for the Wally Shaw info & will post ASAP.