If it supports DLNA or Chromecast (Chromecast as of MediaMonkey 5) you can stream to it from MediaMonkey (Tools > Options > Player).

If it supports Bluetooth you can play to it, if your PC with MediaMonkey can connect to Bluetooth speakers.

If you're going to hardwire it to the PC (headphone/SPDIF out) it would act as an Audio Device on your PC and MediaMonkey can play to it.

I was looking at some thig like this, Soundavo NSA-250 Multiroom Music Streamer Amplifier 2 x 50W, the key is that what ever i use i would like to be able to control it via MM desktop and MM mobile

Define effective? Anything will do, match the power of the amplifier to the speakers power and that's about it.

Do consider,
- Connections, so that your PC can connect to it
- Streaming, Bluetooth, DLNA and Chromecast are great features to have for added flexibility. Both MMA and MM5 would be able to stream via DLNA and Chromecast to it.

What is the most cost effective WIFI Stereo Amplifier that can use to power some passive outdoor speakers. I would like to use MM to control it. Thanks for all help