"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.
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.