How many channels total are you running?
The M5010's just have a line out so you feed the exact same signal to another amp. This is generally not something you want to do (have more than one speaker in the same room playing the exact same signal as another).
What speakers are you driving? Unfortunately speaker specifications for power are notoriously bogus as a rule! I'd bet your front speakers cannot handle 500 watts continuous, but could handle 500W for some short duration (like a rim shot). The amps are generally spec'ed with continuous or RMS watts, so you don't need a 500W amp.
However, assuming the rest of your ducks are in a row, I would suggest as follows:
Assume you mostly listen to music (with the drummer mention).
Accordingly, assume you mostly listen in stereo (since that is the format of most music).
When you are listening in stereo, your AVR is only driving two channels and for most speakers, it should be sufficient.
However, if your speakers are "power hungry", I would suggest you get a pair of the Outlaw 2200 monoblock amps to cover the main L & R speakers so you have plenty of juice for stereo, try your Onkyo amps for the rears and surrounds (75WPC is probably enough for them),and use the Denon for your center channel (which may deserve more than 75Watts). If you have additional channels, use the Onkyos first. if you feel like they are lacking in power, then you can add a 5000.
Understand that the 6400 has a very good amp section and the 5000 is probably not much stronger, but splitting out the other channels from the Denon will make a difference.