I don't know what a 500 sounds like compared to a 600 because Robb (Hutzal) won't let us listen to his 500 .

However, I can tell you to not even consider multiple 400s. Although they are great for music, they won't rock you during movies.

The 500s won't be difficult to place in your room. It looks like you have lots of space. You will have to experiment. As for the sub cables, these are not too unsightly and if you have the space, you can tuck them under the baseboards like I did.

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