Speakers don't have any watts. There is a range of power that they can accept. How loud a speaker plays with 1 watt of power at 1m from the speaker is the "efficiency" of the speaker. That tells you how much power the speaker needs but it also depends on the room, how far from them you sit, and how loud you want to go.
So a 50 watt stereo amp will always be able to play a 150w speaker but not to the full volume the speaker could play with more power.
The quality of the sound is separate from the quantity so you can get better sound from 50 than 150 watts if the 50w amp is of higher quality and the speakers are good enough to let you hear that difference.
Power is made up of voltage x current so you can have two amps with the same rating but if one has very little current compared to the other the high current amp might sound more better depending on the impedance of the speaker.