Sub for Quad ELS-989

I have come into possession of a pair of Quad 989's and really love them but when I play music with any medium not to mention strong bass characteristics the protection circuitry kicks in and down goes the volume.
I thought maybe a sub might help this if I ran it through its crossover and filtered out the lower register but then I have a slow driver with very fast electrostatics. Is there a sub as well as a configuration that would help my situation?