American vs Mexican

That could be either the pots of the pickups.

"Tex Mex" pickups were (are?) actually a different series of pickup altogether. Not sure if they ever used them on basses. I've only ever seen them on Stratocasters. As far as I know, they were the same as standard single coils in the MIM guitars, except wound a little hotter

 

the current american standards have the custom shop 60s and they are sweet, however the mexican standards have a pretty weak bridge too so you may consider changing that also

mine has schaller tuners, bone nut, cs 60s pickups, and a vintage style threaded bridge now
i'm really pleased with the the 90s mim bass, and it cost me way less than a 2013 usa jazz

 

+1 I don't know where he got that from? I would have to know more information like what kind of strings you are using? your amp setup? it sounds more like an EQ problem than pickup problem also have you had the bass setup properly?

 

they are spendy . . . your money is better spent elsewhere aka dimarzio (model j / ultra jazz / antiquity II)

 

yeah, the pickups and the bridge were a great combo, without the bridge change the bass still sounded kind of muffled and lacked highs

bought the pickups new at sam ash, you can buy them cheaper now that they have been around a while or maybe even get a pair used on tb in the classifieds

 

check out this video

it gives you an idea of some different pickup choices

also jazz basses that aren't wired correctly or have cold solder joints do some pretty weird things

 

Tex Mex were only for guitars, I thought I saw a tele set before but they are definitely for strats.

OP, your tech is a goof, ignore him. Though I don't advocate against replacing them, they aren't terrible and they definitely are not tex mex pickups. I would recommend you change your pickups before changing your bass, pickups cost under $100 whereas even a used MIA costs at least $700

 

Like Jason said it could be a bad tone pot? I would replace that first and maybe throw in a Grease Bucket circuit I have customized many MIM's into something special I love them but the key is knowing what sound you want and not spending a ton of money on them.

 

don't know what it is, but supposedly it makes it possible for you to use lower tone settings without your bass getting boomy, which can happen with vintage basses

 

A bad or disconnected cap? A MIM jazz should have plenty of warmth.

Tons of people gig with them stock. The pickups aren't awesome but they're not bad. The tex-mex suggestion is babble.

 

Try adjusting the pickup height. It makes a big difference.

 

If the bass plays well (neck, action, frets etc.) it will be an excellent bass. (like mine). And yes the MIM pickups really are nasty. And yes changing to MIA pickups will make that bass come alive. I put SCN pickups in mine and it was like night and day. Of course as usual with Fender when something is too cool they feel obliged to discontinue it. In the day those were the pickup in the MIA Jazz. I don't know what they put in there now.

For my taste the ultimate pickup change would be to Nordies ($$$) but I went Fender SCN because people said they were nearly as good but much cheaper. Now I don't know what the choice would be. Just figure what kind of jazz tone you want ask the guys here for opinions.

As for the volume full up thing, to fix that you have to add a "treble bleed" capacitor on the volume pot so that it doesn't roll off highs when you turn down. It's simple. You can look up wiring online.