Hmm, that's interesting -- while walking back and forth a couple of times between computer and living room I noticed the speakers sounded kinda harsh when I was standing in front of them but sounded much smoother when I was sitting in a normal listening position.
OK this is going to be embarrassing...
As you know I've been working on the room acoustics a bit, and I had improved things to the point where I felt the weakest link in the system was now lack of amplifier power.
There was a bit of an "edge" to the sound when playing at higher volumes that I didn't remember hearing on the system I had back in the good old days (with a big-ass Marantz power amp) -- but I wasn't sure whether to blame lack of amplifier power, abused ears, abused tweeters (nothing like port abuse, don't even start) or something else. The off-axis harshness I noticed when walking around the speakers only added to the confusion.
Didn't feel like spending a lot of $$ on an amp until I knew it would make a difference, so I picked up a used Adcom GFA-555 II tonight and hooked it up. Hard to find the right words but I could finally turn the volume up as loud as I would ever want for normal listening without the system starting to sound strained (I always backed off at that point). So far so good.
The intermittent "edge" on the sound still persisted, even at the same volume levels as before, so I knew the amp wasn't the problem. That left tweeters or ears, and I knew that replacing ears would hurt, so conclusion was "hey now I have an excuse to buy a couple of those nice v4 tweeters". Obvious first step was to make sure that the new tweeters were compatible with the crossover in whatever version of M60 I had (thought it was v2 but wasn't 100% sure) so I grabbed a flashlight and looked at the back of the speakers. Music was still playing.
So I bend down and peer behind the right speaker and first thing I notice is a buzz coming from somewhere around the speaker terminals. Turns out my speakers are M60ti's, which have two pairs of terminals joined by metal strips in order to allow bi-wiring or bi-amping. The nuts on the lower binding posts (where the banana jacks were plugged in) were tight, but the nuts on the upper binding posts were loose. Bzzz bzzz.
Tightened up all the nuts (they were loose on the left speaker as well) and holy ^$%&^% the "edge" on the sound went away, even when walking past the speaker. The sound was harsh off axis because the buzz from the back of the speaker wasn't being blocked by the speaker cabinet.
D'oh !!
