Bright highs on my first build - swap the tweeter or fix the crossover?

Chased almost the exact same ghost with a pair of two-ways I built about six years ago. Harsh sibilance on certain female vocals, cymbals that made me wince, but plenty of tracks sounded lovely. I nearly bought a fancy replacement tweeter before a measurement app on my phone plus a test track with sine sweeps showed me a fat bump right where the crossover handoff happens. The generic network I'd copied had been designed for a diferent driver's impedance, so the level around the top was pushed up several dB. What fixed it for me was cheap: I bought an adjustable resistor and swapped values around til that bump flattened out, then soldered in the closest fixed ones. Total cost was less than a nice lunch. The tweeter itself turned out to be perfectly fine.

The reason tracking albums got worse than others, in my case, was simply that the problem material lives in a narrow frequency band. Songs with lots of energy up there expose it, quieter mixes hide it completely. So the patchy behaviour you're describing doesn't point to a bad tweeter at all, that woud usually sound harsh on everythign.

Before spending money, play a sine sweep through the whole thing and listen, or better yet measure with a cheap mic and some free software. The peak will announce itself. Copying a crossover aimed at different drivers is such a common trap and almost always the answer to mystery harshness in a first build.

I'd try the cheap fix before any of the bigger moves. On my second build, a three-way with a similar layout to yours, I had the same story with harsh vocals and I was convinced the tweeter itself was junk. What actually fixed it was knocking the tweeter level down. I put a couple of resistors in series ahead of the existing network, just cheap 10 watt jobs from the parts bin, and dropped it a bit at a time until the edge came off. Took an afternoon and about the cost of a coffee. The reason this worked, I think, is that the crossover sheet I copied was written for a different mid with a hotter top end than mine, so the whole thing was running bright. Yours is a copied design too, so there's a decent chance the tweeter level just doesn't suit your actual drivers. One thing to watch with your layout is that the mid cabinet sitting right under the tweeter can throw reflections back into it, and that shows up as a narrow spike on exactly the sounds you're describing. A bit of felt or foam around the tweeter face is another free experiment.

If padding it down fixes the fatigue but dulls the rest, then you know it's a balance issue, not a broken driver, and you can decide where to spend money with actual information instead of a guess.

Before you order anything, borrow a mechanic's stethoscope or even a length of clear tubing and hold it against the cabinets while the harsh bits play. On my first three-way the 'spitty' top end turned out to be the little mid enclosure buzzing where a brace had come loose. Sounded exactly like a driver problem through the speakers, but the tubing found it in about a minute. One bead of glue later and the cymbals calmed right down. Worth ten minutes of poking around before you start swapping parts.

Not sold on the claim that a bad tweeter would sound harsh on everything. My last pair had a tweeter with a geniune breakup ridge a bit above the crossover point and it behaved exactly like yours, nasty on sibilants and hats, fine on most other stuff, because plenty of mixes just don't put much energy up there. Swapped the driver and the sharpness vanished without touching the network. So the patchy symptom doesn't settle it either way. Measure before you buy anything, but keep an open mind about which part is guilty until the graph tells you.

One thing I'd check before any parts swapping is where you're sitting relative to that tweeter-between-two-mids layout. I built something with a similar arrangement years ago and the vertical spacing between the drivers created interference effects that changed with listening height, so a few centimetres up or down on the couch completely changed how sharp the top end sounded. Try different seats, or stand and crouch while a nasty track plays. If the harshness comes and goes with small height changes, that tells you the geometry is part of the story, not the tweeter or the network on its own.

Update: sorted, and it was the cheap fix. Took the advice about dropping the tweeter level and put a resistor in series, tried a couple of values until the sibilance calmed down. Turns out the driver was fine all along, I just had it running hot. Cymbals are listenable again and nothing else lost any sparkle. Glad I asked before buying a new tweeter.