>>My hk will do 120w per channel and that seems adequate...so why get a amp?
In general, if 120w per channel is adequate (which it is for most systems) then you don't need an amp. There are a number of cases (some overlapping) where an outboard amp or amps can help :
1. You have a huge honkin' room and like to play really loud so 120w/channel is not sufficient.
2. In some cases 120w/channel is sufficient but if you run the receiver up in that power range for a long period of time it overheats because all that heat is being dissipated into one heat sink array. This is also why some people go for multiple amps -- a 5 channel and a 2 channel or a stack of 2 channel amps. A few people on the board have had to replace a 5 or 7 channel amp with a stack of stereo amps to get around heat problems. Yes I know this is hard to believe.
3. Also, if you are running up near the power limits of the amp that is where many people feel that you can start to hear differences in sound quality between amplifiers. We're not sure whether this is related to output impedence, "soft clipping", or something completely different but many people do report a significant difference when going from a receiver "driven hard" to a power amp with much more hearoom. You would probably get the same effect from a 300w/chan receiver, of course, if one existed.
Bottom line is that if you are not running near the limits of your current receiver then going to separates probably isn't going to make much difference. If you are anywhere near the power limits of your current receiver then an outboard amp can help.
Note that the easiest/cheapest first step is to offload the mains to an outboard stereo amp using the pre-outs in the receiver, leaving more headrooom for the remaining 5 channels.
I don't know if the processors have additional features/performance or if it's just cheaper to buy a good processor than a good high-end receiver (if you're not using the amps) -- all that stuff has been out of my budget for... oh, the last 20 years
