I have quite a bit of experience with the A&H apps and can give you the following advice (apologies if it sounds obvious)
1. Have your IEMs on a stereo mix if possible. That way you can pan the guitars/vox left or right and keep your bass central.
2. Are you sending any FX to your IEM mix by mistake? Bass could have reverb applied mistakenly? I only have a select few channels saved for quick access on my apps but periodically I will add them all and check to see if our mix engineer might have tweaked something by mistake.
3. Is the gain input for the bass channel on your desk set too high/low if you PFL it?
4. Have you tried plugging the bass into another channel to see if it might be the preamp on your usual channel at fault?
5. Check your GEQ/Compressor on the mix app to see if it might be interfering with your tone.
I hope this helps somewhat. I've heard that earphones with ambient ports sacrifice a bit of low end at the expense of stage bleed, which is desirable for some.