Crank up the action a bit, and those TI JFs feel much better, and a bit of excess relief is good for tone anyway.
Asking TI JFs to go thud, though, is like, well, asking an Alembic to go thud. It's the wrong tool for the job. Jazz Flats are, in my opinion, the roundwound player's flat...while they will do some things typical flats will do, in terms of sustain and tone they're more a match for good rounds than GHS or LaBella flats.
To return this thread to its' original heading, sorry, haven't checked out TIs' rounds yet. I'll direct you to the discussion group of a voiciferous TI dealer, thedudepit.com, where these strings are discussed at length and in detail by many users and a company rep.