What is the frequency response spec for your 1x15" cabinet? Some cabinets just cannot reproduce a low-B as well as (higher) notes. If you play an "E" or higher on the low string does the sound level increase?
Many bassists don't realize that their speaker cabinet's frequency response is already rolling off at "E" and is very much weaker down at "B."
For example: Ampeg's published spec for the SVT 8x10" cabinet shows that it is "flat" to around "A", about 3 db down at "E" and then a whopping 10 db down at "B"! No amount of pickup adjusting, EQ boosting or other tweaking will get your speaker to reproduce a note outside of its range.
If necessary, have an electronics tech "look" at your bass's output on an oscilloscope. That will help you know if the problem is with the bass, the amp, or the speaker cabinet.
