Don't look at the power, Slim. Look at the max output SPL. A machine can take more power but it may be more inefficient and hence that power is being wasted as heat.

EP500: 108 anechoically and 118 in room

EP800: 114 anechoically and 124 in room

If you stack two 500s, you'll get 111 anechoically and 121 in room.

If you stack two 800s, you'll get 117 anechoically and 128 in room.

The same analysis can be done for the 600. Of course we know these specs might be wrong, so it's best to send an e-mail to Ian and ask for his advice.

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