I've got a soundstream 10 inch sub that's 800 watt rms I was wondering how big of an amp I should use? I listen to rock so I just want some added bass. Thank you

First you need to know if it's 800 watts rms at how many ohms. Then find an amp that will put out 800 or as close to 800 watts rms at those ohms. You will want a D series amp. Soundstream makes some awesome amps. I use the Tarantula series amps from Soundstrwam in car and I have yet to have a problem with them. Good luck!

If I use a 400 watt amp will that cause any problems?

As long as its 400 watt rms at the right ohms it should be okay. But the speaker will not performing to it's full capablities.