Recording vocals: resonances around 300hz

that's a typical problem in a small recording room.  I would suspect floor - ceiling reflections causing this, potentially enhanced by an even distance between mic-floor and mic sidewall (according to your pic). With wall distance you could play, but since your room has a really low ceiling, there is not much you could do about it, other than moving upstairs and recording in the living room (which may have better acoustics than this hard walled small basement room)