You might have a whisper of a chance at overwintering dahlias in zone 6 if they're up against a foundation and you mulch heavily. A dryer vent in the area doesn't hurt either. Otherwise, you may as well plan to get new dahlias every year.

The tubers cut up from each clump really don't take up that much space if you have just a few plants. Maybe you have a friend or relative with some non-freezing storage that you could put a box or two in for winter?

You don't have to bring them inside as live plants and keep them living all winter. Most of us let them go dormant and store the tubers in the corner of an unheated basement, garage shelf, or similar dark and cool place. From your post, it sounded like you thought maybe you have to keep them living as houseplants all winter? Forgive me if I read you wrong.

nope - you are correct _ I thought I had to keep them living all winter---tubers?

Good call rose nutty. Starjr you should read some about digging up and storing for winter. A great starting place is here
http://dahlias.net/dahwebpg/CultureIndex/Culture.htm
Unfortunately, they look to be revising/updating that site and winter prep, digging, and dividing is absent right now. You can read about the process of overwintering whole clumps of dahlia roots or cutting into single/small clumps at almost any dahlia society website. Do a google search.
Below is another good learning site.

Here is a link that might be useful: Mid Island Dahlia Society