I haven't had them in the same guitar, but in my experience the Riff Raffs are not that much brighter than Mules, just "different" brighter. For example, the Mule bridge gives me a hairier, rougher bright... where the Riff Raff gives me a tighter bright... if that makes any sense?
However, I can tell you, that if you reach the conclusion that Riff Raffs are too bright - then Mississippi Queens might be as well. I have had both of these in the same guitars, and the MQs can get brighter than Riff Raffs.
I've always understood that maple bodies are brighter than what I'm used to, so I'm kind of out of my comfort zone making any definite suggestions for your guitar...
Now, if you had a mahogany Explorer :lol: (I have, and it's got MQs at the moment - lovely. It's had Riff Raffs as well, lovely...)
Welcome to the forum! :D