Echoing what a few others have said, many combos amps in general have a strong, baked-in voicing. Whether that voicing is to your liking or not is entirely up to your taste and needs. The Effects Return on the Rumble 500 (a feature on many larger combo amps) will let you bypass the EQ section of the amp entirely, giving you few variables when tweaking the Helix preamp.
My $0.02 and plenty of people here disagree with me, but having owned the combo and standalone cabs from this series as well...my real beef with the Fender Rumble V3 series was the speakers. There was a very pronounced mid-scoop to them that couldn't really be "dialed out" IME. I mention this only because you brought up wanting flatness/tweakability and intend to use the Helix with your setup. If you hook them all up and love it, more power to you and rock on. Just something to keep in mind during your tonal journey
My $0.02 and plenty of people here disagree with me, but having owned the combo and standalone cabs from this series as well...my real beef with the Fender Rumble V3 series was the speakers. There was a very pronounced mid-scoop to them that couldn't really be "dialed out" IME. I mention this only because you brought up wanting flatness/tweakability and intend to use the Helix with your setup. If you hook them all up and love it, more power to you and rock on. Just something to keep in mind during your tonal journey
