Likely culprit for harshness at higher volumes?

Given that your preamp has a headphone jack, my suggestion is that before going much further you invest in a pair of headphones. Considering the number of variables and possibilities that have been cited, I think that comparing results between headphones and speakers would be very useful in distinguishing between source material or front-end issues, and room/speaker/amplifier issues (despite the fact that headphone listening is a fundamentally different experience than listening via speakers).

A good fairly low priced choice would be the $84 Sony MDR-7506, which is widely respected within its price range. I have a pair that I use for non-critical purposes, primarily for monitoring while doing amateur videography and voice recording, and I've been pleased with it, especially considering the price. I haven't tried it in my main system, though.

04-22-11: Meiwan
At higher volumes you may be noticing harshness in your system that you don't at lower volumes (even though it's there).

This is an important point to keep in mind. As volume increases, the sensitivity of our ears to high and low frequencies increases, relative to their sensitivity to mid-frequencies. That is known as the Fletcher-Munson Effect.

Regards,
-- Al

My best guess is it is the CD player and maybe amp. I doubt it is the speakers or room. Higher order harmonics are not genrrated by the room. You can take steps to improve room acoustics, but that is more like icing on the cake and no way to fix bad equipment. When i first started with CD, my first impresion on 90% of CDs was things were way too harsh at even moderate volume levels . I found the only real way to improve things was to get CD players with way better digital filters and analog stages. I made the natural progerssion with better players and noticed a big improvement with each step. I went from Pioneer BPC, to Theta, to Krell, to Wadia, and back to Krell. :)
you might look for any reviews on your amp to see how it does higher order harmonic distortion. I know Adcom only really made one amp that was even close to "high end", that being the model designed by Nelson Pass. The rest of Adcom amps are strongly mid-fi and not much more than BPC. (sorry to say)

Hi -- thanks for continuing to check in on this topic and offering your suggestions. I am continuing to experiment, mostly with fabric (types and locations) for reflections, speaker location (separation and proximity), analog vs digital interconnections and various types of music (classical, jazz, vocal; low (ie - bass, sax), mid (vocals / guitar) and high (some vocal and violin) pitch sounds across various volume ranges.

I've been spending a lot of time listening, moving stuff around (including using a different room and moving components between rooms) and characterizing. :) This seems really more of an art than a science.

I've tried a lot of things and I think I'm reaching the limit of room dynamics and my components without buying stuff (dampeners, drapes, different components, etc).

While it's not perfect and some harshness is still there, it's definitely better than a few days ago and I'm actually able to turn up the volume quite a bit more before the harshness becomes unbearable. Again, before, I would get unbearable harshness before the sound was uncomfortably loud. Now, I can get the sound to be uncomfortably loud before the harshness is really unsettling (and perhaps I'm also now reaching the limits of my speakers and/or amp). The clapping / pinging test is much better now, especially in corners where it was the worst.

I appreciate the suggestions and understand the reasoning for a power conditioner and the headphones (actually, I really want to buy some nice headphones and using them for additional testing here is an additional reason). And I keep learning new things: I had to look up the Fletcher-Munson Effect (never heard of it before). BTW, what is BPC (as in, my Adcom amp are not much more than BPC)? Not that I disagree but just not sure what it stands for. :) I think my GFA-5400 is rev 1 of 2 versions (I have the rev made in Japan, not China).

But, given my budget, I can almost buy a used CD player that may help improve a weak link in my system (access to additional bits and higher sampling rates); I think my current DACs (in both my AVM2 and X111ES are 8 or 16 bit and 48 or 96 KHz). Perhaps getting something like a used Cambridge Audio CD player in the $200 price range would be the best use of any budget I might have.

Or maybe I should just be happy with what I have. It doesn't actually sound too bad now after trying some of the inexpensive suggestions received; it's not perfect but better. But I can see how a budget could very quickly spin wildly out of control here. :)

Your Adcom is having difficulty with transients. The poorer the recording, the more apparent this becomes. Try a really good one like say, Lyle lovett's "Natural Forces" or Jimmy Roger's "Allstars", or Ray Charles' "Genius Loves Company". A really good amp won't be limited by the recording in that it will simply reproduce what's there accurately. So if the information is diminished, it won't attenuate that upper mid-range already attenuated in the poor recording we're all familiar with as "harshness". The recording will simply be apparently "lacking". If a great recording eliminates the issue, you need to consider the perfomance of your amp. All KEFs are great speakers regardless of there particular shortcomings. If it hurts or is bothering you that much, it's the amp.