that's pretty small, but i'm in a similar boat, working with "what i've got".
it's hard to say if your booth is too 'dead' or if reflections are cancelling some frequencies without hearing a recording. can you post something up to listen to?
i'm guessing by loss of crispness you mean that some frequencies aren't being picked up accurately. try moving the mic around and see if you can bring some of them back. if so then you may still have some nasty reflections in such a tight spot.
I intend to start doing vocals this spring in a closet and i'm uncertain how the results are going to be. In the meantime all I know is that it's the most isolated spot in the house and I'm hoping the right amount of aborption is going to make it work.