Sub bass frequency' s are many tens of feet long, so 5ft from the speakers shouldnt make any difference.
In my opinion I would only use sealed for subs and you need drivers with huge magnets to control the cone, lack of control leads to boom in my experience and I cant see how a passive radiator is under control properly?
crossed over low enough it maybe ok, eq will probably help too, Your room will also benefit from the flanking subs as the room modes will be lessened and modes normally lead to one note boom.