Just to add one more factor to the mix (don't hate me): these three strings are so close (Perms, Bels and Passione - and I would also add Jargars) as being great orchestra strings etc, that one thing I found as a factor/variant was the endpin I used with them.
On both the Upton and Argentine bass, a steel endpin was a better fit for the Bel Cantos. A carbon fiber shaft noticeably weakened the volume.
The Jargars and the Passione's did better with the carbon fiber than steel rod, but best with the Atomic wood/aluminum pin.
There may be some relationship between the composition of the string and that of the pin or it may just be my imagination... But I did want to throw that in. Generally, the Atomic (also KC Strings wood/cf pin, too) seems to make the strings feel as if they have less tension.
Louis
Great point. I have just been A/B'ing some arco strings and other issues aside, the endpin material/mass/stiffness has a pretty amazing effect on the sound and response. I swapped between stainless steel, regular carbon steel, and steel tube/carbon fiber/epoxy rod I made, and titanium. All 1/2" rods.
With different strings, I left the same strings on and tried all four pins. Totally different sound on all strings, different bow response, and differing low-end volume and "bloom."
Lots of variables...
For me, right now, best I have found is Belcanti with titanium pin; clear and singing up high, organ-like in the lower register.
So, not only does string performance/experience vary widely from bass to bass, but also from endpin to endpin...