IMO you wont find a single 400 dollar tower speaker that will sound better then those Martin Logans, let alone a pair. You will read over and over again on this forum that if you have a decent set of bookshelves and a good sub woofer crossed over at the right frequency, it will sound just as good if not better then a pair of towers.
You already have an awesome set of book shelves in those Martin Logans, and while I'm not familiar with Dayton audio, I'm sure your sub is no slouch. If you spend 400 dollars on tower speakers for the front you are going to be stuck with a pair of surrounds that sound better then your front speakers, which makes zero since.
My advice would be to listen to zieglj01, jump on another one of those lx16's while they are still in stock so you can have 3 voice matched speakers across your front end. With your budget it is going to be very hard for you to find another speaker later on that will match up with ML's tweeter. As far as surrounds go there are tons of options...