I play a EBMM Sterling 5HS and a Sterling 5H in much the same situation. I find MM basses have a very high output.
You could, first of all, try replacing the battery.
Start with all bass EQ controls flat (center detent). I always run full volume on my bass.
Then I would look to gain structure. At your regular gig, it could very well be the settings on the bass channel strip. If the sound person is not necessarily a pro, or knows little about what they are doing, bass can be a tricky beast to properly set gain levels and EQ. For me, I had to go very simple, Bass > Markbass Compressore > Radial JDI. If I try to use any type of preamp, my channel always ends up distorted.
While on the subject of gain staging, also check the gain on whatever interface is between your personal monitor mixer and the main mixer monitor sends.
Next, work on your personal monitor mix. There is plenty of material available here on TalkBass and searching the web for setting IEMs. A quick set-up would be to turn all the channel sends off, bring your master up to say 1 o'clock, bring your bass control on the monitor mixer up to about 1/2 (noon?). With still only your bass in you ears, adjust master to where the bass is loud enough for your liking. Then bring up only the other channels as you need to hear them, keeping the unimportant ones low.
When I get done setting my IEMs, usually the bass, drums, and lead vocals are pretty hot. Everything else is mixed fairly low. Usually if I have trouble hearing the bass, something else is too loud, or I do not have my in-ears in my ears properly. I tight seal is a must, especially for bass players. Make sure you use both ears as well.
The Older I Get,
The Less I Knew