That's a great point, and having an electronics background, it was the first thing that I considered.
I actually did both of your suggestions - I had the salesguy come to the house, and his reaction was "Wow! That sounds awful!...there's definitely some weird bass anomaly going on here." (At least he was honest about it.)
We then lugged the whole system back to his store and set it up in his much larger and more symmetrical listening room, and it sounded fine. So I think we could safely eliminate hardware failure from the list of causes.
I don't think any individual component in my system is at fault here; I'm pretty convinced that the hardware system (in total)is just a bad match for my particular room. With that as a given, the question becomes one of practicality - how do I achieve a more listenable system in my environment at minimum additional cost?
For the folks who suggested replacing the speakers, my only comment is that I picked the Thiels because they were the only speakers that I heard in my price range that reproduced music the way I heard it live. I listened to Vandersteens, Maggies, Paradigms and a couple of others, but in my opinion, none of them came close to being able to reproduce the nuances of a guitar tone to the degree that the Thiels did.
If I can retain my system's ability to resolve that level of detail, but push it "back" in the presentation and balance it with a stronger midbass, I'd have it made.
Thanks again to all who responded!!
joe