Many players, especially here on TB, are willing to look 'under the hood' for possible solutions to tone issues. For those who are, here are a couple of potential fixes to the 'muddy' bass and related low B problems. First we have to acknowledge that, to some extent, these problems are a matter of personal preference. This is evident from the comments here, with some critical of the Rumble 200's performance and others praising that performance. For those who have a Rumble 200 (or one of the other Rumbles) and find that the bass is muddy or over prominent, but are otherwise pleased, a couple simple mods may prove helpful.
First, stuff (Dacron, or fiber glass: not tightly) the bass ports. This damps the Helmholtz resonance of the cab making the loading aperiodic. (tightly stuffing changes the cab to sealed). The effect may be adjusted by changing the amount of damping material in the ports. Many will find that this is all that is needed.
If the problem is still evident, a blanket of damping material may be fastened directly being the driver. Remove the 15 inch driver and set the cone face-down on a flat surface. In the past (when drivers were rear mounted and cab backs were removable), this was done by using wood staples to fasten a blanket of acoustic damping material over the back of the driver. For front mounted drivers (like most modern cabs) the blanket can be fastened by flipping the edge of the blanket back toward the opening and stapling. Size the blanket by draping the sheet of damping material over the back of the driver and cut the sheet, allowing a couple of inches beyond the frame for the fold-over. Cut a small slit for the speaker wires. A potential downside to this mod is some loss of (reflected) mids (and aggressiveness). Try a little boosting of the lower and upper mids.
These mods, combined with some EQ experimentation will probably resolve the bass issues for many. Both of these mods are inexpensive and easily reversed. In the event that some are still unhappy with the bass tone, it is probably best to simply try something else. For those enamored of the otherwise positive attributes of the Rumble 200, a driver upgrade, such as those described in the Rumbles thread, may provide what they are looking for.