Sub for single-driver desktop speakers

Hi there audio friends. Can you suggest a musical subwoofer for use with my new desktop speakers?  I just got the Arche Audio FR2 single driver speakers, they sound good, but I would like some more low midtone/bass. They only go down to 100hz. I don’t need a big powerful sub, just something to extend the range down to maybe 60 or 50 hz, to add a little «soy sauce to the rice», so to speak. The sub should not interfere with the purity of the single driver design.