do you have room for 5.1 or 7.1 set-up???? it would be best to start there, because mostly any 'good' sub will over power that or any sound bar.
I would keep away from Yamaha speakers, they make great receivers (I own 2) but speakers you can do better (sound bars could be diffrent) & you don't have to match brands when it comes to Subs eather. For downfiring subs 'OutLaw' makes some of the best Subs. + Make sure a downfiring sub is good for your room conditions, too.
I would like to buy a subwoofer to pair up with Yamaha's top of the line single speaker system. If I buy a Yamaha YST-SW1500, which has 1000 watts of power, will I overload the YSP4000? Am I going overboard by buying this powerful of a subwoofer? If so, which one should I go with?