Likely culprit for harshness at higher volumes?

As a solution put ferrite claps on the interconnects from the preamp to the amp.
get four snap on ones from Radio Shack and put one at each end of the interconnect from the preamp to the amp. about five inches from the RCAs at each end. (you see ones similar on various computer cords now and then)
If they are too loose when clamped down, wrap some tape on the wires to give the wire body enough volume to keep the ferrites from sliding around too much.
The Ferrites should help with the HF glare.

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?