I've used the sub channel (mains small) to power the built-in woofs but it doesn't work so well. As MLS has often said the RS1K powered woofer is not a subwoofer. I want to keep the LFE stuff out of those woofers. I'm guessing from what jester has said even if you could hook up all three sub outputs the two unbalanced sub outs would go dead as soon as you hooked up the balanced sub. Would be nice to know for sure though.
What would be really cool is if the three sub outputs could be independently assignable. My problem is I want Lexicon/Meridian/Tact functions at Outlaw prices.