At the moment I have two 12"hx2's on a RF T600.2 there getting 375 each and sharing a 4cu box tuned@38, question is would it be louder to run one 12" on 750w in 4cu?
No it wont be any luder. I do know the power hx2 series likes a bigger box (2.25-2.5) but you are massively underpowering them. Only way to get louder is to get a bigger amp.