I have D'Addario Chromes on my '64 TB II and I sure recognize that sound (and like it). But then Rob - together with Fräulein Rommel - belongs to the diehard "we hate flats" faction.
This is after all how TBirds sounded when they came out - that and a little flat of course.
I utterly detest flats on two fronts -
Tactile, I just hate the way they feel on my fingers, just awful.
Sonically, they suck the life out of a Thunderbird - for me at least.
When I bought my '06 Studio it had flats on it, talk about beating dead meat, that poor bass sounded so bad! I installed a set of SIT Powersteels on it and the TB Plus just came to life!