If you use the preamp output of the Rogue to use another amp the amp section of the Rogue will have no load. Tube amps need a load connected. If you use a subwoofer with an integrated amp you will have to run the speakers full range without the xover option. SVS has a 45 day return policy so you can try a sub and see if that does the trick!
https://www.svsound.com/pages/subwoofers
https://www.svsound.com/pages/subwoofers