Your article on canna lilies (from the Adams Co. office) mentions nothing about keeping the lilies potted. Is it OK to just cut them back and put the large pot in a dark cool place?
Centre County Pennsylvania
It is best to remove the bulbs from the soil.and store on racks or mesh bags so that air can circulate freely among the clumps.
The following publication from Purdue has more information:http://www.hort.purdue.edu/ext/HO-085.pdf