Mesas are know for being very smooth. The Recto cabs are loaded with Celestion V30s, which are by no means fizzy, they're supposed to be very fat and warm.
Is this cabinet brand new perchance? If so, the speakers are probably also brand new, which means they need to be broken in. After a while you'll hear some of the scratchiness go away and the sound will round out if you beat them over the head with some volume, but if the cab is used, someone might have put different speakers in it.