[Mike P.]

if having a sub this big would over power them

You adjust the gain control so the sub output balances with the other speakers.

 

[Mike P.]

Some people say it is better to have the extra power in case you ever need it

That's called headroom, always a good thing! Since the sub designs are sealed I'd go with the xls400.

 

[Mike P.]

Any sub/room interaction issue can be dealt with. It's a matter of determining the issue and applying the solution.