BINGO

The rear speakers will only be getting 1/2 the power that you were getting before. The powered subs will boost the power back up for it's own need ... but ... subs suck at pumping out mid to high range. You'll be getting plenty of base but you'll prolly hafta tweek up the treble a bit to compensate.

Are you starting to see why sound engineers get paid so much out in Hollyweird?

BTW, I'm not sure what you mean by RCA cables. Typically audio signals are on wires with RCA connectors and line signals are on wires with mini-phono jacks. The connectors make very little difference on resistance until you get up into VHF & UHF (i.e., shielded cables).