How to connect a pair of Bagend 18 subs to Krell K


I have a KRC Preamp and a Krell FPB 300. I use XLR cables to connect the Pre to Power. I now want to connect a pair of Bagend 18 subs to the system so how do I do it. The Bagends do not have any XLR input or output.
Please advise as I am at a complete loss.
I have tried a number of ways but it does not seem to work, it ends up with the left channel having no sound. The system sounds fantastic without any subs connected.
The speakers are the NHT 3.3 Thank you
sugar72
Use the RCA outs on the preamp directly to the subs. Let your mains run full range.