I have yet to really play much with this extension. I bought it souly because I am gigging around Texas this summer and some of the locations will not have great in-house sound. I have tested it out a bit at the house and without changing basses, turning knobs, or even messing with volume, the rig sounds three times better with the extension. I did quite a few A/B tests to it and the rig as a whole beats out the combo by itself in every way.
I own the Rumble V3 500 210 combo. I warn people often that it's a little too high pitched and bright. The extension completely fixes this issue.
My only complaint is that the combo is not the exact width of the extension. It looks a little less professional to me to have a smaller box on a bigger box.