I was just setting here when I suddenly recalled something that I had forgotten about. Many years ago, I found a local small retailer who was closing his store and had a (Parasound I believe, don't remember the model) power amplifier he was liquidating. After confirming that I could return it if I wasn't satisfied, I bought it.
I don't remember what I was using for preamplifier, possibly Adcom. After getting it home and setup in my system I discovered that the treble frequencies were not well represented. This was a very warm amp with great bass grip but missing much of the treble. I ran it for a week or so then returned it.
This experience would seem to indicate that an amp can make a difference in what is and isn't heard.