Your sub needs upgrading no matter what. Trade it in. Don't think twice about it.

Even with a sub, you will get more emotional impact setting the cross-over for the M5s to 40Hz. With the M2 you'll have to cross at 80 and M3 at 60.

BTW, I run 4-channels (fronts and sides). I have sealed subs on every channel. My 800 is dedicated to LFE only. There is no substitute for full range on every channel.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated