I wouldn't say Boomers are bright?
it all depends on what you consider "bright". the term "brightness" is used in many different ways and understood in different ways, because we never speak in terms of frequencies.
let me explain:
we start to talk about "brightness" up from 300/700hz, where we start to enter the "hi-mid" range, the one that gives the "snap" to the overall sound (think about the nasal middy sound of Jaco), "clearing" the notes, giving you more separation between them.
the higher you go in terms of frequencies, the more you reach the metallic sounds, up to the "chainsaw" rasp, like, for example, the Stingray sound of Tim Commerford in the first RATM album.
some consider "bright" a string with lot of snap on the hi-mids, because the sound is clearer, and some consider bright a string with lots of metallic sound.
also, the lack of other frequencies makes them appear brighter, for example when you have less low-mids, the hi-mids and highs step out more.
Boomers are not "metallic", but they are, in the overall sound, bright and well defined, keeping also great low-mids to meat up the sound.
I'll add another thing always forgot: there are frequencies that we only hear playing alone, but are useless when playing with other instruments, because they get lost.
having too much highs is beautiful when you play at home, alone in your room, but in the exact moment the drum or guitar enter, you don't hear them anymore, and only hear the center frequencies of the bass (200-1000hz appoximately).
too much bass frequencies go with the bass drum and toms, the highs with the cymbals, so in the mix you wont really hear them, and if you lack in the "mid" your instrument gets lost.
a well defined, middy bass sound is "brighter" and can be heard more than a metallic scooped sound, which sounds brighter alone, but too bassy and lost in the mix.
a Precision is less bright than a Jazz because of the hi-mids, not the highs, and the Jazz is brighter because, using both pickups a slight low-mid scoop is engaged.
a Stingray is brighter because it's centered on the hi-mids, lacking the low-mids and giving it the "pointy" sound.
if you put some GHS on a Stingray, it's still bright, but gets some "meat" on the low mids, making it sound fuller.
they are anything but "smooth" on the surface

I love Rotosound, but wont suggest them to anybody searching for "smooth surfaced" strings.