The 300 watt number may be the power draw from the AC. Usually written on the back near the power cord.
You need speakers that are 8 ohms or higher and that are rated to be used with amplifiers from about 20 -100 watts per channel. Speakers have an efficiency rating that tells you that if you put 1 watt into the speaker it will play this XXdb loud at 1m in front of it. The higher the XX number the less power you need to play louder.