Yes, burn-in can reduce initial apparent brightness over time.
Yes, the latest crop of tube amps (as a group) are less warm & syrupy sounding.
Yes, system-matching has a large effect on bright vs not-bright sound (brightness can be added or subtracted by canny component choices & matching).
But I’m a little surprised that no one here touched on a very relevant fact of audio life: the ever-present RESOLUTION vs MUSICALITY debate. This takes place in two intersecting spheres:
1 - Design/Build/Manufacture: Some audio designers & companies are known for emphasizing resolution and detail retrieval ("plankton") over all else. Considering that all audio gear gives, at best, a carefully chosen version of how music sounds IRL, this tendence to chase resolution is a legitimate choice...though one that disagrees with me and has since my youth (not a new thing)
2 - Audio Consumers: Similarly, individual listeners also have preferences in the gear they invest in. Some audiophiles go crazy for highly resolving, forensically detailed gear, while others go in the other direction, towards "musicality" (by which I mean similarity to the sound of music IRL), which includes a natural weight in the upper midrange and a slow roll-off in the treble, matching what you’ll hear in any good concert hall
In my experience, a lot of brightness is actually dialed in, selected by both parties in the audio hobby. My preferences take me in the other direction and always have. I’ve heard so much live music of all kinds, and other than some amplification anomalies in live electrified shows, I can’t think of any music events that were consistently bright in the way certain components and speakers are.