I was really close to replacing the speaker on mine, the reviews are favorable; but ultimately decided not to roll the dice on the mod and just get another amp. I ended up with a Markbass 121p and its head and shoulders above. (I still like the rumble for low level jams, but it has limitations.)
I was also really close to a GK MB amp, 112 or 210 (though these have really gone up in price.) Ampeg Rocketbass would be my recommendation if you want to stay in a similar budget.
My experience is similar. The Rumble 100 that I had was not clear to my ear, no matter what I tried. I sold it and purchased Markbass Little Mark 250 Blackline and a Markbass New York 121 cab. Def an upgrade for me. Best of luck with your decision.