Denon 1712, Onkyo NR609 or RC360, Pioneer VSX-921, Yamaha RX-A700.
If all your devices are HDMI look at the Denon 1712.
If you have a Wii or other devices with component video then perhaps one of the other 4 receivers.
If you want networking and internet radio streaming get the Onkyos.
You'll have to provide way more info in order to us to recommend a specific model or models.
All those receivers will power the BS21 speakers fine.
My first two choices would be the Denon 1712 and Yamaha A700.http://www.amazon.com/Yamaha-RX-A700...1829721&sr=8-1