[quote name='Dad3353' timestamp='1407252852' post='2518815']
The experts and pundits will rip into this, I don't doubt, but, for my tuppence-worth, I think it's the impedance change that gives the extra watts (and so volume...). It will only be really applicable to ss amps, as you intimate; valve amps won't give more power per se, but the better dispersion will give a better 'spread' of the spl generated. An ss amp rated at 200w into 8 ohm, and 350w into 4 ohm will be able to get louder with 2 x 8 ohm cabs (and so 350w...) compared to with only one (with only 200w available...). The difference will only be a couple of db, though.
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This would be right if you're using identical cabs. But if not, then it can vary greatly.
An efficient single 8Ω cab (e.g. my Markbass 2x10) could be very much louder than two not very efficient cabs (e.g. my 2 old Marshall 1x15s at 4Ω).
Having 1 efficient cab has many benefits. Having 2 gives you even more db if needed.
It's not so much about how many watts there are, but what a cab can do with the signal provided.