I reviewed both the MB15 and the 115MB some time ago, but it basically comes down to this: That box contains a PS15, runs flat down to 300 Hz, where a broad rise begins. The rise peaks to +2 dB at about 130-150 Hz, drops to zero again at 100, and then finally heads south at 12 dB/octave. The cab/driver is a bit "shouty" in the low mids, and there's at least one surprisingly strong wolf note I could identify. Lining the cab with that 2" foam sold by Parts Express made a big improvement, however. The cab comes stock only minimally lined.
The cab needs LOTS of up-eq in the low bass region, and by that I mean 50-80 Hz. If all you have for a bass control is a shelving one, though, feel free to use it, because any boost it applies in the subsonic region will be handled well by the "big driver in a small sealed box" configuration -- specifically, the air cushion it presents to a driver to delay overexcursion. The driver+box can handle a maximum of 130 watts from 90 Hz all the way down to about 18 Hz. The beauty there is that a pair of 115MBs will absorb the full power of a 250 watt head with virtually no chance of damage from overexcursion, no matter how irresponsibly the amp's bass controls are used (and/or how irresponsibly an active bass's bass control is used), and no matter how loud the amp is run. In that regard, the MB15+115MB, or a BX250 with a pair of 115MBs, would be the perfect stage rig when you have to share with another bass player on the same bill -- even someone with a 5-string. Or a 115MB pair with any other 200-250 watt amp for that matter.
One 115MB alone, even lined and eq'd as carefully as possible, is still a bit "barky" and impolite with significant amounts of lo-mid glare. Doubling up on the foam lining might help -- but after all, the cab was built small because that was Job One for this model. (It was designed, above all else, to be little. Perfect acoustics was a secondary consideration.) What really helps, though, is running a stacked pair of them. They tie in well down in the bass region to produce a far more weighty (summed) sound, with far less emphasis on the mids than you get with just one cab. A pair, tonally, sounds like a whole different amp.
Hope this helps.