Is there anyway to convert a JL Sub to connect to Main speakers?

I am thinking of going straight from a Lumin X1 Dac/Streamer etc. directly to my main mono block amps, thus eliminating a preamp. I was very impressed with the purity of the sound  from the Lumin when running direct like this. However, I would like to incorporate my subs.

I have been told by Lumin that using the line level outs (XLR) for the main amps and the (RCA) outs for my subs at the same time will result in not so good of sound. I tried it, and though it works, the sound is not as pure as just using the one set of outputs for my mains.

Lumin suggested using Rel subs connected to the speakers/amps using the high level speaker connection.
But, I already own 4 JL Audio F-113 subs that only accept line (low) level connections.

Question:
Is there some type of converter that can adapt a line level input  from the sub to connect to the high level speakers?
Sort of like converting my JL Subs to the Rel design.

ozzy