Likely culprit for harshness at higher volumes?

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.

hat 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.

You may have a burnt voice coil that is rubbing - possibly one or both speakers are damaged. What you describe is definitely NOT normal.

It's possible that your speakers have a distortion which is present but below the detection threashold at lower levels, and then as you go up to high levels, the distortion becomes louder than the decection threshold. In other words, your ears may have a non-linear perception of what is fundamentally a linear distortion.

It is also possible that your speakers have a distortion or coloration that is level-dependent, or non-linear. For example, if the voice coil of your midwoofer is heating up faster than the voice coil of your tweeter, then your midwoofer will compress more at higher levels. So you could end up with a brighter tonal balance at high levels.

My instinct is that the first situation is the case, this based on my work with horns. In effect a Uni-Q driver horn-loads the tweeter, but much about that "horn" is not optimum from a distortion-minimization standpoint. We see this in prosound with a sub-optimal horn: It sounds fine at low levels, but crank it up and it sounds harsh. The distortion was there all along, but we don't hear it at low levels.

Imo, ime, ymmv, etc.

Duke
dealer/manufacturer

Its difficult to know anything by the volume references you have included. Typically, you want to report loudness in decibels (ie. 80-90-100, whatever) via the use of an SPL meter to better report how loud you are talking about. Since you like the Shack, they have them in there for pretty reasonable prices. If at some point you are going to address room issues, having even a moderately good SPL meter is very handy.

Your speakers may or may not be the cause, remember the speaker is conveying all the problems from the source on through to it. Lots of areas to pick things up and be exposed by the speakers at higher volumes.

Do a simple clap test in your room, without music on and listen for the slap echo, etc. . . a lively room will really accentuate such problems. I would do this and then address some reflectivity with items around the house for experimentation (blankets, pillows, speaker location, etc. . .

Good luck and don't start replacing too much until you at least look into the room to some degree, it could save you a fortune and lots of headaches.

Hi folks, thanks for the continued comments.

So, if I summarize correctly, the primary suspects include:
1) speakers (distortion)
2) amplifier (insufficient to drive the KEF 4ohm load)
3) RF noise entering the lines (will get the ferrite clamps to check this out)
4) room dynamics (reflections); I walked around the room quickly and clapped in various locations. It was interesting -- even though the room is fully carpeted, I could hear reflections and a kind of pinging after each clap (I don't really know how to describe it -- a kind of reverb echo).

Oh, yea I know that the volume level reference I made above is a little abstract but I assumed that you experts would have an order of magnitude sense given the components I'm using. :) Not extremely loud but louder than conversation levels.

I'm going to try some of the easy suggestions next (this weekend):
1) using the KEF Q10's again; do a larger sample to see if perhaps my C75's are blown (but by nature, the KEFs may be subject to the distortion I'm observing in which case I may hear no difference; in this case, may need to borrow some speakers)
2) use some blankets to minimize reflectivity
3) use the ferrite clamps
4) Not sure what I can do to check if for amp insufficiency; I can try using my Nakamichi AV8 receiver but I'm assuming that the Adcom is a better amp.

Anyway, I'll start w/ #1,2,3 and go from there. I'll report back later.