Pickups, you notice most when you change in spec. Like if you go from a single coil to a split coil. Or you go from ceramic to alnico. Or low output to high output. Or active to passive.
When you make a change to a different pickup with similar output and specs but supposedly of higher quality, that can often be so subtle that it’s not worth it.
Say you have tonal requirements that your current pickups weren’t designed to meet. Then you change your pickups to those that have your preferences. Then you’ll be happy. But say that you like vintage style P pickups and your current pickups are vintage style P pickups with the right output and alnico magnets, but you just get seduced into thinking that a boutique hand wound vintage style P pickup will transform your bass into something much more wonderful. Yeah, then you shouldn’t be surprised if you can’t tell the difference. You might be able to, but you also might not.
But I think usually people are pretty happy with pickup changes because stock pickups are designed to appeal to the masses and meet a certain price point, and then individual players agonize over what to install before they purchase aftermarket pickups, so chances are good that the pickups are closer to the desired specs.
Preamps, too, are a bit like that. Spec changes are noticeable. If you have a preamp which is designed to be neutral and have unity gain at noon with a certain amount of headroom, and you buy another preamp that is kind of the same in that regard, I wouldn't expect it to blow your mind. But if you change a preamp that has no headroom to another one that has tons, you'll notice the difference. If you change a preamp that inserts gobs of gain to another one that is unity gain, of course, you'll notice. If you change a preamp that is designed to be neutral to one that is designed to put a little character on your tone, you'll notice. If you change a preamp to another one that has a fundamentally different idea of how to adjust your tone, then you'll notice.
But like, if you swapped a bass with an EMG BTS to a Nordstrand 2B, which are both relatively neutral sounding, clean, 2-band EQs with relatively normal choices for EQ changes, I doubt I'd notice the difference until I looked under the hood.