Michael, as Alan indicated, you may not be able to take full advantage of the maximum loudness capability of the EP800(unless you simply play it as loud as you can, without regard to correctly balancing with the M22s). In any case you, not the sub, are in control of its level, so it can't automatically "drown out" the M22s or any other speaker. If you want, you can turn its level down to zero.
My view is that I'd suggest using two EP500s rather than one EP800 and placing them so as to minimize the affect of room modes on bass smoothness(typically at the midpoints of opposite side walls or front and back walls, or in diagonally opposite corners). Although on some material the deeper extension of the EP800 would possibly be of audible benefit, the two subs would provide more than enough maximum loudness capability and have the advantage of greater smoothness of response when properly positioned.
Enjoy the music, not the equipment.