D'oh, it's late. I read the title as "I re-TUNED" [as in, altered the ports/box-tuning of] my Rumble 500 (which people have done with some of the Rumble combos -- don't know about the 500) and kept looking for info on how and to what effect.
IME, that combo can sound great but can also get boomy in the upper bass/low mids. My feeling is that the way the cab is tuned contributes to some of this, but I can't say for sure. Rolling the mid bands up and treble and bass down a bit does give a more neutral tone profile to my ears but, with some basses/in some rooms, dialing the low mids back a bit (and possibly using the bass knob to balance that out and dial in the overall amount of low-end presence) can help clear things up.
That said, you've decided it wasn't for you and returned (not re-tuned) it, which is fine. In similarly sized combos the Markbass and Ampeg combos mentioned are worth a shot (different vibes -- from each other and from the Rumble) and I suspect that the new GK 212 combo would be a winner (I haven't played it but, based on my experience with other GK stuff, would expect it to be more clearly present in a mix than the Rumble).