It is very difficult to give any guidance with out reading the document  involved, i.e., the lease. I am assuming first that your lease allows a sublet.  Second that the "reasonable effots" language that you state comes from the lease.  But what I do not know is if the lease requires that th landlord sign something allowing the sublet or something else in there that could cause you to be in breach or the sublet to be void. I am thinking he has to legally consent - i.e., in writing - and that you would want him to to protect yourself. With out it you would be liable still under your lease.  His refusal could be a breach and that is an issue to use in trying to have the lease declared void.  Good luck.