Hey, so glad that you're liking them so far! They are not stiff at all, so yep, you might need some time to adapt your touch and bass set-up to it. But I'd say that if lack of stiffness is your only complaint, you've found a worthy keeper.
Regarding to the buzz on the bridge for the A string, I'd really need more info. What I can tell you is:
- I've never had or heard about TI flats coming defective out of the box (I'm sure some do, but what I mean is that the issue is marginal enough to almost discard that option).
- TIs have fair less tension than the La Bellas you came from, so a truss rod tweak (in the loosening direction) is a must.
- Also, being less stiff (=more floppy), this strings have more travel distance than stiffer ones, which usually means that, apart from the truss rod tweak, some height increase in the saddles might be necessary too (I'd say that, given the same attack, stiffer strings allow for a lower action).
So maybe the buzz you're hearing comes from the end of the fretboard? I'm just guessing.
- Lastly, ofc, you can double check that the ball end of that string is tight against the end of the bridge and that the saddles, screws and spring are all tight and properly set-up.