No, I don't think you would reap any benefits from changing to RCA. Generally speaking, the high level signal coming from your amp will integrate the subs better than a line level out of your pre... the sonic signature of your power amp reaches both.
Some high-end sub manufacturers, such as REL, specifically recommend using the high level (speaker level) input rather than line level in.
Some high-end sub manufacturers, such as REL, specifically recommend using the high level (speaker level) input rather than line level in.