That is why we are here, to discuss

not just follow like sheep
Based on a couple of reviews on the 4600, its supposed to put out ~138wpc in 2 channel mode and ~40wpc all channels driven. I recognize that the all channels drive test is not realistic, however, it is a metric nonetheless.
This link, while incomplete and not very up to date, has amp tests:
http://www.geocities.com/Area51/Hollow/3401/ratevsac.htm
Your Yam, 4600 is not tested but I couldn't find a Yam with such a disparity between 2 ch and all channel driven as you claim.
Do you have a link to the test?
Yes, it is a metric, but if it has nothing to do with reality, what good is it for?
Given how strong the 4600 is in 2 channel mode, I'm not expecting any material improvement when adding an Anthem MCA to the 100s.
Same or a lesser amp
However, where I would expect to see improvement is when listening to multichannel music and movies, where the 4600 is driving a total of....yes....17 different drivers (5 tweeters, 6 mid-ranges, 6 woofers) in my 5.1 setup.
Just because you unbundled and count all the drivers, another metric, it has no relevance to the capability of the amp driving a multi-way speaker.
My local Paradigm dealer also carries Anthem and has a demo MCA20 on hand. He's been very accommodating and will let me take the MCA20 home to test it in my own environment.
Just be careful how you compare or you will get a false sense of security
Here's a question...how would one set up an A/B for a 4600+ speakers vs. 4600+MCA20+Speakers. I guess, I could have one set of speaker wires run from the 4600 to one set of binding posts on the speaker directly and then another set of speaker wires from the MCA to the 2nd set of binding posts (without removing the connecting plate between the two sets of binding posts) and then have the Yammy Pre-outs run to the MCA.
NO!!! This is how you will blow an amp for sure.
You would have to have a double pole switch at the speakers with speaker wire from the two amps to it on one side and to the speaker from the center pole to break before you make contact. You would need to sources to drive the amps so you can level match them to very close tolerances. However, you will not be able to synchronize two CDs an you can swap Cds or use two without back and forth switching without stopping or muting a CD or you will figure out which CD is behind the other and will know.