Hi, Dougie! Show us a pic of your Rumble and join The Club.
First of all, I agree with your assessment of the 4 ohm mode with just two Rumble V3 112 cabs.
But I disagree with your assessment of the 2 ohm mode. The 800 HD will still output up to 800 continuous watts in 2 ohm mode with a 4 ohm load connected (just two 8 ohm cabs). [I dunno what, if any, disadvantage there is to setting the 2 ohm mode while using only a 4 ohm load.] You should connect, at least, three 8 ohm cabs to correctly use the 2 ohm mode, thus giving each cab a max 266.67 watts continuous which should be fairly safe for the 112s.
Hope this helps.
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