Well...
The day I bought my '78 Fender P-Bass (summer of 1988), it had Roto 66's on it. Before I went to bed that night, it had a set of Roto RS77LD's over top of what little was left of those Fender frets. And, it wore them until1994 or so. They only came off because, good as they sounded? That sound wasn't
quite the sound I wanted from that bass. But, if what you want is a generic, thumpy "P-Bass with Flats" sound? The Roto's will definitely give it to you. I also had them on my '18 Gibson Thunderbird. Killer tone on a T-Bird... I sold that bass, but not because of the tone. Long story...
Galli Jazz Flats? I have those on my '20 EBMM 4HH Stingray Special; I like them a lot - on that bass. On my P-Bass? Nope; not the tone I want from that bass. But, if you wanted a "medium dark" tone, with good low-mids; mids; and some useful highs? They'd be a good choice... I will say that they are definitely the slickest, smoothest flats I've ever played. And. in 50+ years of playing flats? I've played a lot of them...
La Bella DTF's? The 760FS's are darker than the Roto 77's, but not by a whole lot. If you want "Old School Thump"? The La Bellas will give it to you - as will the heavier gauge GHS Precision Flats. Speaking of which... the tone I realized that I wanted from my P-Bass - after trying several kinds of flats - was the tone of a brand new, fresh from the factory '78 P-Bass. With the foam mute in the bridge cover, over a set of the factory light gauge Squier flats. The closest I can come to that - barring a pricey collector set of NOS Squier flats? A set of GHS #3025 Precision Flats, under the foam mute in my P-Bass's bridge cover. Which, while it
is the authentic P-Bass tone, is, surprisingly, not very "thumpy" at all. There is a very good low end, but there's also very good low mids and mids. The top end is... well, it's there, but... only just.
So... bottom line? If you can deal with their bridge cable stiffness? Go with the Roto 77's. If you feel like you don't want to? The Galli Jazz Flats would be a good - but a little more mid-rangey - choice...