Yo Ashes of cold:
Q-man is very right. Mixing with only cans does not give you "what you hear" when pushed down to tape or whatever.
It's an anomaly. The environment of sound within cans, especially good ones, treats your ears and the cut appears to need little adjusting. Then, when you put it to tape and play it back, you will find the bass is thin or heavy, the lead vocal is covered with reverb, etc., etc. ect. to paraphrase the King and I.
You can give it a college try until you get some monitors. You might get lucky and get a good mix. But, when you get your monitors, you will set up your mixes with them all of the time.
Green Hornet