Seriously, leave the sub in the box, you can lay the on its side, and slide it up the stairs, you'll need to push low on the bottom of the box so it doesn't try to tip and catch the corner of the next step. If your stairs are carpeted or at least have a smooth surface on the front edges, this should work with out the risk of dropping it and give you a fairly convenient way to catch your breath as/if needed. Still doesn't hurt to have a second person, but they don't need to be weightlifters - my GF can hold it still if I need to regrip or get thirsty and need to go grab a beer or something.
Note: If you take it out of the box, it will be less than 20" long. My steps measure 12" apart, so it needs to be at least 24" to insure the front edge of 2 steps are supporting it at any time (so it doesn't tip into the face of next step). The box is just over 25".