Chris,
I want to make sure I am on the same page as a possible solution. Control and power breaks down like this.
You should not use all of those style/color tags when you want me to answer questions in a specific quote/reply style format; it makes a mess in the reply/html window.
I assumed you would use two of the Yamahas for the front end; one for Left and Right and one for the Center, leaving an unused channel. Though, if you use only one sub, you could use that spare channel for the subwoofer, obviously..
The P5000S for only sixty more dollars than the P2500S is a heck of a bargain and of course you should go for it. It's also a most unusual bargain; the price difference is normally two hundred dollars.
The P5000S will barely draw any power at idle, and it uses the same basic design as P2500S as you correctly assumed. It just has more output power(a good thing).
The DCX has 3 inputs and 6 outputs. Each one represents one channel. So you would use two of the inputs for L and R from the receiver pre-outs. The 3rd input would be used for the C pre-amp signal from the receiver. This leaves you with 6 outputs from the DCX. Use two for the L and R, one for the C and two for the L and R subs if you use stereo subs, or one for the sub if you use only one sub. Of course, if you go stereo subs, you will need to have another amplifier in order to drive both of the subs. You don't need an Art Cleanbox that another poster mentioned, as the DCX has a clean input pre that will boost the signal accordingly. You may need an ART DTI box if you end up with a ground loop between the DCX and receiver, however. The DTI is a passive interface box that has ground isolation by way of a transformer. It also converts the unbalanced receiver signal to a true balanced line. So ideally, you would use a really short(like 12" or shorter) RCA patch cable from the receiver to the DTI input side, then a standard XLR patch cable from the DTI output to DCX input. Each DTI has two channels, so you would need two DTI boxes if you also route the center channel to the DCX.
-Chris