First, since you don't say whether your sub is a powered monitor or a loudspeaker I will assume that it is a powered monitor. As a match to this type of sub you cannot go wrong with any of the HS series monitors from Yamaha. If you are looking for something a bit more economical I have also used the KRK Rokit8's ad the Presonus E8's and found both of these to be of excellent sound quality for the PSR keyboards. Of the three, I have found the Yamaha's to be the most accurate to the instruments intended sound, while the KRK's and the E8's lean more toward embellishing the lower frequency's. While you are shopping for monitors always keep in mind that these are defined as "nearfield monitors". If you are looking to play in larger rooms, venues etc, you may want to consider moving up to powered loudspeakers.
