Amplifier to Drive 4 Speakers (2 Left Channels & 2 Right Channels)

I currently have a Yamaha RX-V995 receiver connected to 4 outdoor speakers. Speaker terminals A are connected to Left and Right speakers on the patio. The speaker lines go through an impedence matching volume switch. Speaker terminals B are connected to Left and Right speakers by the swimming pool. These lines go through another impedence matching volume switch. I would like to add a passive subwoofer to the speakers by the pool. The subwoofer I have in mind has speaker line inputs and outputs, so I could route the existing speaker cables directly to the subwoofer and output them to the Left and Right speakers (like satellites). I'm not sure the 100 watts per channel of my receiver will provide enough power to drive all five speakers. Is there a reasonably priced amplifier I can connect to my receiver that will power all of the speakers? Basically, it would need two Left and two Right channels so that it would behave as my receiver does using the A & B channels. Or, could I connect a subwoofer using pre-outs and then run the main speaker channels through an A/B switch to the speakers?