The only difference is that for headphones you can find measurements that on top of that are more easily usable ie the variables are easier to control. You won't find these for speakers since the room affects speaker response curves more than headphones will.
You'll just have to go purely by ear. Look for a sinesweep vid that has time stamps for the frequencies and listen to where it's much louder or too quiet, especially where there's a relatively sudden and very noticeable change in loudness somewhere in the sweep. Adjust those, repeat the sinesweep, if it sounds good try it on music then adjust again. For example if there's a dip in the bass and you boost there then you get boomy or boomier bass then instead of boosting where it dips cut it around that frequency instead.
If you want a more accurate picture you can get a good quality USB mic, mount it where your head would be, and run an RTA program. If you know anybody who's into car audio, ask for their help as they might have such a laptop set up already or have a DSP unit they can easily detach from their systems and then you just bring that into your room along with a car battery.
