Are certain bass strings louder than others?

It's also dependent on the magnets in your pickups.
Once I had a bass with AlNiCo magnets in the pickups that was noticeably louder with Cobalt flats over Nickel rounds.

At the time I had another bass with the same pickups, only difference was that they had Ceramic magnets. In that case there was not a big difference between Cobalt & Nickel, as far as output goes.

 

I haven't noticed any trend where one brand of strings are consistently louder than other brands when used on the same bass.

Again, I haven't noticed any significant difference between SS and NPS in overall output when used on the same bass.

That said, some alloys, such as Alloy 52 (nickel-iron) and Cobalt alloy, can have a higher output due to their magnetically active properties, while pure nickel can be a bit more laid-back and quieter in comparison.

As far as I can see, strings are only a small part of the whole equation; the chances are the pickup types and configurations and the way the bass is set up would have more bearing on the overall output than the strings themselves.

 

I agree with what has been said above.

In my experience, it is sometimes the opposite. It depends on a lot of factors, including how well the strings are made, but the pickups height will have much more effect.

 

All other things being equal? Yes, some strings can, indeed, be louder that others. Why? The gauge of the string (as in size of the inner core), and the alloy that it and the outer wrap is made of. In searching for the "Right" string for a given bass - and with 24 of the things, that's a Lot of searching - I have come across some strings that were louder on a given bass than others. However... while it was sometimes noticeable, it was also pretty subtle. There may be strings that are noticeably louder than others, but I haven't found them - yet...