You need to choose your speakers before you settle on an amp. Some speakers require more power than others, so the best amp for you will vary depending on the speakers you will use them with.
I am about 95%/5% music/HT, so for me, it would make sense to put my serious money into the front two speakers and insure they had good solid power while saving money on the surrounds. You only mention replacing the front three, so I wonder if you are thinking the same way. If you intend to keep your surrounds and plan to have a 7 channel system, then buying something like the Outlaw 5000x is a good option. It is a good value on 5 channels and more than ample power for your DefTech 800 surrounds. For the question of whether or not you should use the 5000x for your center, it depends on the needs of your center, but would do fine with most. However, I am not sure you can find a better 4 channel amp for the price of the 5000x, so I still think it is a great option.
You did not specify your current AVR, but if you got the 5000x, you are now looking at the AVR to only drive the front two (or three) channels.
The 5000x is an update of the 5000. Outlaws claims the same performance (so I assume it is internally the same design). The difference is that the 5000X adds XLR connections!
On the speakers, I really hope you are looking at the Canton Chrono SL 596.2! The .2 is important because it has the ceramic tweeter that (at least in my subjective view) presents an outstanding
high end. (and also Accessories4Less is currently throwing in a free center channel which essentially makes it the same price as the 596 without the .2)