Have you ever tried the Steve Harris Rotosound flats? Those supposedly have very high tension but they are thicker than normal so that might turn some off.
I've used these and honestly they felt equal to or less than the regular Rotosound flats that cost less than the Rotosound Steve Harris set. In my recollection the bottom 2 strings are the same gauge, the G and D are both larger than the standard set by .05, might be Steve uses the thicker girth to round out the G and D because of the Fender's placement of the G and D poles on the P-bass pickup? No clue.
I used the Steve Harris strings once, then went back to the standard Rotosound strings.
I find the Roto's have a bit more upper mid grunt, GHS Precision flats have a warmer bottom and a hair more sustain than the Roto's, and Fender flats are kinda even across the board when it comes to flatwound tone. I used D'addario Chromes once, and found them ok at first, but they feel really scooped when they break in, they retain more treble than other flats i've played but they lose mids, and I find mids are a must-have in bass tone and I can't live without them.
I currently have Roto's (their standard set of flats) on my NS20005, and Fender Flats (50-105, one of the new sets with green silks) on a non-Spector, single-pickup passive bass. I really liked the GHS flats too, smoothest nicest feeling strings I've played, but to me they go a bit "thud" after a while. In defense of the GHS, the Roto's do eventually wear out of that tasty high mid grunt as well, but I find they pair with humbuckers a little nicer.