Just changing the 12 speaker may or may not do the job. If the new speaker isn't happy within the dimensions of the enclosure it might not sound as good as your current speaker.
My guess is that it might not be a volume problem but you're pushing too much low end into these cabs. I'd try a high pass filter, like the SFX Thumpinator, to remove some of the very low frequencies that the Streamliner is undoubtedly trying to generate. That should stop the distortion, clean up your sound and improve your tone.
It's a shame you're not using a TC RH* amp or another amp that goes to 2.67 ohms, then you could just add another RS112, to make 3 cabs. If you're pushing the 112 into distortion I'd also look at an RS210 or another RS212 to pair with your current 212.