Not much to add to what's been said already -- you should be able to make things work with your current rig -- but I do see one problem with running the two cabs. MB heads are 4 Ohms minimum, correct? Is your 410 8 Ohms or 4 Ohms? The 210? Unless both are 8 Ohms, you won't be able to run them together with the MB head. If they are both 8 Ohms, the 410 and 210 will be unbalanced, with the 210 getting more power than it should. In that situation, you'd be better off trading the 410 for another 210 and stacking vertically.
Check EQ for sure and change strings if necessary (my main bass has rounds, but I've been able to get across fine with flats, too -- rounds will give you more mids and highs, but not all flats are the same -- TIs and GHS Precision have more mids than some)
Are you happy with the guitarists' sound, btw? Two way street, though we rarely treat it as such. Not saying anyone needs to get meddlesome or pissy -- and maybe they're offering you valid feedback -- but the sounds need to work together.
EQ. Strings. Trade the 410 for a 210, pocket some change, stack vertically. (If you like how the 410 works on its own, you don't need to get rid of it -- you can keep one big and one small cab, maybe tilt them back on a wedge -- but, since you can't use the two cabs simultaneously, it makes sense to me.)
If that doesn't get you where you want to go, then we can talk about everybody's favorite rigs.
Good luck!