Dan - normally you could use the pre-outs on a receiver, but your model RX-V481 does not have pre-outs. It is actually a pretty low-end receiver. Like tls49 said, it's rated for 80 watts per channel into 2 channels. When all channels driven, it can drop to something like 40 watts. When your doing a HT movie this can become critical because the receiver can start clipping/distorting the sound because the power supply is not large enough to drive the speakers. This can result in burned out tweeters and damaged voice coils.
You could do something like this:
It's not optimal, but if you really want to continue with your current receiver, it's your only way to add an external amp. There are cheaper speaker-to-rca convertors, but they are usually limited to 50 watts / channel. It's up to you.
If you're playing at the level of Monitor Audio Silver, you will probably want to get a more reference receiver or amp anyways.
This is not the same thing as bi-amping.