Connecting Subwoofer’s to Willsenton R8

I have been wanting to add 2 Rel S/ 510 or 812 subwoofers to my system. I have a Willsenton R8 (upgraded tubes) , newer Cornwall 4s speakers. I am confused with the many solutions I have read and even some say you can’t add subwoofers. I also have a McIntosh C2500 tube preamp running a separate system . If anyone can tell me in simple terms how or if this can be done. 
thank you