AV Receiver w/ amplifier

Hello. Thank you for the add. I only understand the basics of the A/V Receiver world. I have a couple older A/V receivers. The first is Yamaha HTR-5730. The second is an even older Onkyo. I recently upgraded my home theater to a brand new Yamaha. I was wanting to use one of my old receivers (mentioned above) with my old Cerwin Vega V-10F tower speakers. The ohms seem to check out on everything to work together. Problem I am faced with is that these are being used at our farm, in the barn, when we are outside. If I turn the volume up over 50% they will eventually shut the system down. Seeing how these speakers are way more wattage than the receivers, I figured I would just buy an external amplifier to get them the juice they need, I only need a little more volume, really just need them to stop going into protection mode after a long period of play. I also installed a 70 cfm external fan on the amplifier to help disperse heat on summer days. I cannot seem to figure out how to hook one of these receivers to my amplifier. The Onkyo only has a single pre-amp out, while the Onkyo M-282 amplifier has a right and left in. If I use a splitter, am I losing "stereo" and only getting mono? Where is the pre amp outs on the newer/older Yamaha?