I would think daisy chaining Rumble combos would work without a problem.
In fact, owning both a Rumble 100 and 500 combo, I decided to just give it a try. Here some crappy smartphone pictures:
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What a beautiful sight, right? ;-)
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The J going into the Rumble 100 and a send out...
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...into the 500C.
It seems to work great. I also compared the volume levels of the 500C directly plugged in and as a return from the 100C. I couldn't hear much difference, if any at all.
The only thing to keep in mind is that from the return path to the speaker, there is no control (like gain, EQ, the vintage button nor even master volume) at all over the signal at the receiving amp so all the settings have to be done on the sending amp. The sending amp works as a pre amp and the receiving just as a power amp.
As mentioned earlier, this makes sense if you look at the diagram and see where the return signal taps into the signal path:
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