Likely culprit for harshness at higher volumes?

Using the KEF Q10's didn't seem to make much difference; noticeable less bass but the harshness is still there, especially when a singer hits loud, high notes. Perhaps the KEF Q10's are too similar to the C75's?

I hope the Monster MC 200I-2M I'm getting next week to connect the pre-amp to the amp will make a difference.

I'll need to research what ferrite claps are -- I've never heard the term before. But sounds easy enough. I can try it w/ the current interconnects and when the Monster MC 200I-2M arrive.

Thanks for all the comments. Still open to any suggestions until the issue is solved. :)

I'll try the ferrite claps and report back later.

http://www.radioshack.com/product/index.jsp?productId=3012599
Called 'snap choke core now.

Add a subwoofer for better foundation.
You are probably boosting the volume to get more bass
and this is overdriving the other frequencies.
This used to happen to me.

I am turning up the volume a bit; not necessarily to get more bass but to hear some of the other instruments that may be in the background and/or a little quieter.

I found that on some CDs, at background listening levels, sound seemed to come mostly from one side or the other. When turning up the volume a bit, I'd then notice additional instruments coming from the other side (mostly some sort of percussive instrument). Since then, I've gotten used to playing CDs at a certain volume levels.

So, the 'desired' listening levels may be a little loud but I don't think they are overly so -- I'm not concerned about damaging my hearing at these levels (but perhaps my ears are already bad :)).

But if certain frequencies are being overdriven, what's the solution? Get different speakers? If so, any specific speaker attributes I should be considering? Larger tweeter / mids? Bi-wire capable? Capable of higher power inputs?

Or is this an amp issue? Need a more powerful amp?

Sounds like (excuse the pun), it's an issue with the amplifier. Your Anthem is a pretty good pre, but Adcom power amps are not that great (having owned one). Seems like at low volume the adcom lacks detail and dynamics - you need to crank it up to get anything out of it. It may be having trouble driving the 4 ohm Kefs. You may not necessarily need more power, just a better quality power amp.
Best idea would be to borrow or try another/better power amp and see what happens. A good power amp will have lots of detail/dynamics at lower volume levels.