crackling noise at high volume

"Also, has the 4-5 times I've heard this crackling done permanent damage to the speakers?"

Probably not, at least not yet, as things appartently sound right at medium loud levels, but if it was me, I wouldn't go past the 11 or 12 o'clock position as you might be starting to drive your amp into "clipping" once you past that 12 o'clock position and "clipping" WILL destroy tweeters.

Now if you routinely like to "crank it up", then you might want to think about getting some speakers with not only higher sensitivity (87 db is probably a little low, you might want get speakers with 90 db sensitivity or better), but also speakers that have can handle higher power levels.

If they don't seem to sound different you've probably done no damage, but repeating that exercise will surely kill them eventually.
I've done it twice to my own pair of Arro's, fortunatly with no negative long term effects.
In one instance I was driving them too hard, in another I carelessly switched sources with the volume up(I NEVER did that again!)

Sandman,

Here's an interesting article that explain amp power and how it relates to speaker sensitivity.

http://allegro-sound.com/Power.html

From just reading it, and putting number in the calulator (included in article), that I'm thinking that because of the Arro's 80 watt max program rating, that you shouldn't be driving these speakers beyond 95 db SPL which is loud for pop music, but not necessarly loud for rock music, no matter what amp you're driving them with.

Btw the numbers I used in the calulator were as followed:

Measured Distance from Listener to Speaker - 2 Meters

Desired Sound Pressure at Listener - 95 db SPL

Speaker Sensitivity Rating (1w/1m) + 5db - 86 db

Desired Amplifier Headroom (3db min) - 4 db

which gave me

Amplifier POWER Needed - 80 Watts

Beyond 95 db SPL you're overdriving what the Arro's stated Max Program Rating is.

A related question: Do overmodulated upper midrange notes always suggest speaker damage? I've got two pair of speakers that I've been swapping in- and out for a few weeks now, and they both make the same rattly / overmodulated distortion when playing certain passages of piano music, especially piano music just above middle-C. I don't think it's the recordings because the sounds don't seem to track across the two sets of speakers -- but it does seem an awfully specific type of dissonance to be mechanical failure of one or more of the drivers. Shouldn't I hear the same distortion with other instruments?

For clarification....Tweeters are pretty tough, so it probably is not the music volume per se that is causing the problem. Most likely it is the TOTAL signal level, including bass which never gets to the tweeter, which is causing your power amplifier to "Clip". Cliping distortion generates a lot of high frequency signal which was never part of the original music signal. Cliping kills tweeters.