I am trying to hook up a single MTX 800 watt speaker with an RMS of 400 watts & 8 ohms. What I want to know is... Does the RMS watts of the subwoofer need to match the RMS watts on the AMP? I have found many amps that output enough watts but the RMS is always lower than 400. Will that matter?

It sounds like the peak power rating for the speaker is 800w. While the RMS (normal operating wattage) is 400w at 8 ohms. For maximum efficiency, yes the amp and speaker should match - but it certainly isn't an absolute. If the amp is lower, it can't push the speaker to it's full potential. But if it sounds fine to you, then who cares. Just be aware that an 'underpowered' amp will try to work too hard - so having a higher powered amp is a better idea. This is part of where the term 'headroom' comes into it.
Hope this helps!
Dave
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