Sterling Sub Ray4 on 18 Volts

I wouldn't do it because the capacitors might be only rated at 16 volts. If they get more than 16 they will explode

 

If it’s preamp is a sealed module with no external capacitors and especially if it is the same as the Stingray it probably be fine. Have you checked the EB forum?

 

egads surface mounts and that old noisy TL062. Since you're into tweaking why not go all way since you have a good quality bass to work with. Put in a Bartolini 2 band or similar that won't have a problem running at 18 volts. The results will blow you away. Unless you prefer the sound of the stock preamp

 

My tech upgraded my Sub Ray4 to 18v. He upgraded the pickups and preamp with EMG's.

 

I use A Bartolini 3 band preamp. Two band sound flat to me

 

For the second battery you can't get around cutting into the wood to make space. You can install another battery box next to the existing one or put battery under the pickguard. I put both batteries under the pickguard and used a drill and wood chisels to cut out the wood. The best mod I ever did and it became my main bass. I never cared for 3 bands because its too much filtering.