Apparently the Sting Ray's hotness is a known issue!
Welcome to TalkBass, Ryan.
Other than that, try turning the Rumble's Gain fully counter-clockwise. It won't affect the volume, it's more of a pad to increase your bass signal, which you obviously don't need. Also, there is no "neutral" settings of EQ on the Rumble, 12 o'clock is not flat, they are all active. So turn down the Bass and Treble settings, at least, perhaps the Mids as well. Also, turn off the horns in the combo and cab. Lots of Rumblers do that, just because.
Come discuss in the
Fender Rumble Club and get a free membership. See what the Rumblers there with active basses do.
Good luck. Hope this helps.