A couple of years ago, near here, the DOT soil temperature monitoring station had the frostline 84" deep. Tulips freeze solid every year here. I bought my house in 2003, and I have tulips from the original owner that are still blooming every year. I've never touched them once. Last year it was -38F in January, without much snowcover at the time, and the tulips were fine; even the ones I planted on Dec. 5th with the frost about 2" deep. Tulips want to freeze. That's why they aren't perennial in the South.
I can't comment on potted tulips, but in general, they need to be cold and stay cold.
