Hi, gio992.
Just curious if it’s the same channel that’s failing? Not sure how relevant this is, but could open further (possibly relevant?) conversation.
Speakers have voice coils that warm up when power is sent through them. Sort of like an electric range. The more current, the hotter it gets. Along with the omelette that’s sitting in the pan on top of it. When sustained power passes through a tiny tweeter wire there’s not a lot of headroom (or, forgiveness) there.
The combination of a not-so-efficient speaker and moderate powered amp at high volume levels is, historically, not the best plan for success. Using a car analogy: I like to launch my vehicle from 0-60 in less than 4 seconds. I can attempt this in my wife’s 99HP Fiat 500x convertible until I a) burn out the clutch -- or b) die of old age, whichever comes first. Or do it on the first try with my Shelby GT350. Clutch still fully intact, and I am only seconds older, rather than years older.
What we’re dealing with here is some sort of dynamic compression. You want the music to be hard-hitting, energetic, and dynamic. Something is presenting limitations to the process. Most likely, the amp is just running out of "oomph" and presenting sustained, long duration energy to a tweeter that’s not a fan of long duration energy. Tweeters can survive short term peaks, provided there is sufficient space between the notes to allow them to cool off. One speaker manufacture of a speaker line we sold claimed to send a 400 watt peak into their tweeters to test them. This might just be "technobabble", but it’s not out of the realm of possibility that could be valid if presented to the tweeter for a few microseconds.
Anything you do to decrease dynamic compression will be helpful. Crossovers with "production grade parts" can be a factor. I have not been able to find out anything about the Rega RX5 crossovers. My assumption is that they are using good "stuff" inside. (We’ve had very good results with improving the signal path to tweeters with our mods and getting a much more lively tweeter output -- thus lowering the need to crank up the volume). Higher gauge speaker cables can be a factor in dynamic compression. Dynamics get squashed -- turn up the volume to compensate. Broken tweeters. And last, but not least, I’d take a good look at the power being delivered to your electronics. I found that power delivery upgrades (wall plugs, PCs, and real power distribution vs "surge protectors") helps ALL components, improves instantaneous response and gets you noticeably most impactful output. When you can reduce the volume knob just a tad and obtain the same (or higher) perceived volume level, less risk of burning out a tweeter.
Or, get a bigger amp, and meatier speaker cabling, and improved power delivery just to be safe.
Good luck.