Speaker or line level in?

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.

Thanks Arni.

One more question. I'm not sure how to word this but here goes. Am I losing valuable power from my amp running speaker level inputs? In other words am I drawing 150 watts (for example) from my Mcintosh amp to provide a signal to the sub amps?

Hello???

Anybody???

OK the last question is kind of stupid I guess, but I AM Nuguy not Smartguy. You don't know if you don't ask right?