Seems to be the typical complaint of Sterling by MM pickups.
Most of the issue is that the pickup coils are wired in Series. The SUB pickups are internally wired and cannot be changed without destroying the pickup cover (and potentially the pickup, too). The Ray 34 basses have better components and pickups that have 4 leads and can be wired any way you want.
Check in the control cavity if your pickup has 4 leads. If you can wire it in parallel, it will take some of the edge off.
If it comes to pickup replacement, any of the aftermarket companies offering a MM replacement will be suffice.
SD and Delano, I believe, are the only companies that offer AlNiCo and Ceramic MM pickups. Nordstrand offers the Big Blademan Ceramic.
I only mention this because the original Sterling used a Ceramic pickup. You may or may not want that.
And the SD 3-coil pickup was an AlNiCo version of the original Sterling Ceramic 3-coil pickup. It will
ONLY work in the EBMM Sterling. The body has a special route in the pickup cavity that will accommodate the 3rd coil of that pickup.
If I remember correctly, the SB14 does not have the 3-way switch like the original EBMM Sterling. So unless you install a 3-way switch with the intent of going Series/Parallel/Single coil, the Nord MM4.4 isn’t worth getting. A standard MM pickup (made of two single coils) will be fine.
A very underrated MM AlNiCo pickup on the market is the Kiesel/Carvin HB.
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I have a pair of 5-string HB’s in one bass. The pickup is made extremely well, and in my opinion, even the pole-piece ground wire is goddamn sexy:
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These pickups have a very smooth, full sound wired in Parallel. In Series, they have nice midrange that isn’t overwhelming. When Kiesel installs these into their basses, they wire them in Series. You can search the ‘tube for how they sound.
I am not an endorser of Kiesel, I just like that pickup a lot.