Hey Plucky, are you sure you had enough real-life experience with your Rumble? Two gigs is not a lot when it comes to setting EQ on a Rumble. They might look easy to tweak, but lots of people are taken by surprise by the power of the Gain on those things. Just a little too much Gain can make you sound really boomy, and in fact, Fender made those amps to work with no Gain at all, and I often do, especially with upright bass. If your guitarist was commenting on the boominess of your sound, that could be why.
Only two gigs with the combo, but closer to 20 full-volume rehearsals were done with it as well. I only cited gigs because some people feel that rehearsal spaces aren't the be-all for testing gear, since most rehearsals can be Volume Wars 7: The Reverberation every time.
I always run my amps super clean (unless I want OD, but not in the group I was citing), with the master either dimed or at 3-4 o clock, so overuse of gain was not the issue. The boominess was just a side effect of the baked-in mid response of the speakers, as this issue happened with a stack of the 1x15s as well. Lots and lots of bass, but the mids were far too scooped. The boominess was also noted by the solo artist, a singer, not the guitarist. If even he was noticing something was off, something had to be really off lol.
Not to knock the amps or that particular sound, some people like the baked-in tone of them and that's great. Some people like an uber scooped sound, such as Fieldy from Korn, which is why there are options and EQ knobs, so that people can make that decision for themselves. My issue is when it's baked into the speaker's frequency response, it is far too limiting on the rig since it's like trying to unbake a cake. That's why I feel it's only fair to warn others before they buy that it may not be something that works for them based on everything I've cited.