We are controlling a LifeStyle 650 through an external control system. Is it posible to power on and/or power off the device sending POST commands?
Yes, this is possible posting a press and release POWER key values to the /key endpoint. Note, this is a toggle command; if you need to be sure the device specifically powers on or powers off, you should first check GET /now_playing to determine the product's state, and then sending the POWER key if necessary. You can find more details in the documentation for /key on the SoundTouch Control API documentation page..
Please let us know if you run into any issues or have further questions!