My Roomates Want To Gouge My Rent Because I Want to Sublet One of My Rooms

I recently was one of four guys that signed a one year lease for three floors of a house in Cambridge. The rent is 2,900 per month all together. Upon moving in there were 5 rooms, a living room, a kitchen, and 1.5 bathrooms. One room was huge, two similarly sized and large, and two incredibly small rooms on two different floors. Nobody wanted the two rooms and I drew the short straw. Before drawing straws we agreed on the pricing of all the rooms and I got the two small rooms. We all agreed my rent would be 610 per month because the rooms are smaller and much less desirable than theirs. After moving in I decided I wanted to sublet one of the small rooms. This upset the roommates because I would be paying much less than they would be paying in rent and they decided that if I got someone to sublet for the second room, that they would raise my rent to 900 per month. They are claiming they have the right to do this because the person would inhabit the common areas. I cannot believe the law would allow them to do this and I can't imagine a reason stopping me from subletting out a piece of my space. Does the fact that the sublettor would be share the common area matter legally? What are my options? How can I fight back and stop them from demanding more money from me if I sublet the place. I never got the fixed pricing of 610 in writing either but I assume since four people signed the lease the most I could be held responsible for is 750 per month. Can anybody explain what I should do? I'm not afraid to sue or take them to court.