for a Spektrum, yes, you should use + expo rates.
personally, I'd recommend not using dual rates at all. you don't want to be fumbling around looking for a dual rate switch while you're flying.
(you mentioned the airplane was 'difficult to control' ... was it difficult as in TOO sensitive? (you just touch the stick and the airplane immediately heads in the commanded direction) or was it difficult to control 'not sensitive enough'? )

edit, you DID check the CG.