Coming from owning 2 Fender Rumble V3 cabinets (1x15s) and a Fender Rumble v3 combo, I can tell you that I still own my GK stuff and sold the Fender stuff. My main beef with them was how poorly they would cut in a band mix compared to my GK stuff. GK usually gets the "it's too scooped" stigma, but the EQ on a GK head is very flexible and this can easily be compensated for. The Rumbles sound fantastic on their own when there isn't too much "competition" (in the bedroom, at a guitar store, low volume coffee shop gig with acoustic guitars, etc.). However, throw them in the mix with a full drum set, guitarist with a halfstack, or an overzealous keyboard player and you might be playing Find the Bass all night long.
IME it's because of the tremendous mid scoop baked into them. Now if it was just the head, you could bypass it or run a pedal in the effects loop and bump things up again, right? Well, unfortunately it's in the speakers. The speakers are loud and they do rumble, true to their namesake, but mid retention on them is abysmal. My Traynor YBA200 through the 1x15s was so mid shy that I thought the amp needed to be serviced, that something was wrong with it. Plopped it onto my Kasino 2x15s and bam, there's the mids in full force.
There are plenty of people who love their Rumbles, and like I said they work well in certain contexts for me, but being a GK guy and having hopped into that yard, I'm letting you know that you might feel the same as me if you take the plunge on that. I played a show with the Fender Rumble (two actually) and the guy I back HATED it. He said he "...couldn't hear [me] right" and that it just felt like the bass wasn't there. When I went back to my GK rig, he loved it and told me to never use that other one LOL....so if it's noticeable to a vocalist, I'd assume it's not just my imagination.