I have Rokit10/3's---3 way, 10" woofer. ---They still like a sub! (I added a JBL LSR310S-the S must stand for "sweet"!) I think all near-field and mid-field monitors can use one. The KRK Rokit series are a little under powered. I've never heard the step up-the VXT series. The subwoofer lets the amps in the KRK's work up a little higher with what they have. The JBL pre-set crosses over at 80HZ