I've always stuck with the thought that you need the same speakers (or matched center) with the same amplification for your front three. Are you using the Klipsch for the fronts? Buy another pair to use one for the center?
If you want a 3 channel amp, regardless of power the B&K on audiogon for $500 is great. You could also go with a 5 channel and use it for the rears too. The final choice is to buy a 6 channel amp that is bridgeable to 3 channels. You don't need the power but why waste it?
Personally, I'd say sell the Adcom and go with 1 multichannel amp. You will be happier in the long run. The rears will blend better and the gain to all speakers will remain constant. If you don't have the same amps all around, the rears may not increase at the same rate as the fronts.
Sell the Adcom (nice amp but not multi)this will also increase your budget.
Either sell or buy more klipsch
The Energy is a great sub for the money.