Plug it into the amp? No. Plug it directly into the amp's speaker? Yes, if the speaker is of the proper impedance and can handle the wattage put out by the head. DO NOT plug an amp's speaker output into any input on another amp, though, unless you like spending money to fix or replace your equipment.
A dummy load would also work, yes, but for the amount you're going to pay for one, you'd be better off buying a cabinet and just muffling the crap out of it, since you'll be able to use the cab in live situations, and also be able to mic it up should you decide that the line out doesn't provide the tone you're looking for.
If you're isolating your cabinet by piling blankets on top of it or putting it in a closet, or any other means that confines it to a very small space, make sure that you're not confining the head as well, only the cabinet. Tube amps get very hot, and the excess heat buildup from confining a tube head could trash the amp or, more dangerously, cause a fire. Just use a long speaker cable and keep the head in a open, ventilated area.
As for Amplitube or other amp simulators, I can't help you there. I don't use them, as I'm a miking-the-speaker-cab type of guy myself.