I want to understand this as well from a numbers standpoint. Anecdotally, I have an F6 running off a Focusrite Solo into some 6R speakers with 88dB sensitivity and it plays loud enough to shake my two-car garage with the garage door open, volume knob at 2 o'clock. So I don't think you're going to have any problems.
I would love for someone to swoop in and correct me on the following:
From a numbers standpoint, the way I understand it the F6 is voltage-limited into 8R speakers, but can supply more current for 4R speakers up to that voltage limit. I don't know if the F6 has a max voltage-out that it is capable of with any voltage-in, but my understanding is that sqrt(25W * 8R) = V which puts the output voltage for achieving 25W into 8R speakers at ~14V. If that also represents a max v-out that the F6 can do, that means the max wattage it can put into 12R speakers would be 14V*14V/12R = 16.3W, and I = ~1.15A (which is nowhere near the F6's amperage out capability, so that's leaving electrons on the table). If that's really the case and you find that isn't enough, you could double that voltage by switching to XLR, converting it to a monoblock and building a second one, making it 28V*28V/12R = 65W. At that point it'd still only be using 2.3A so it still wouldn't be current-limited, since the F6 can provide 3.5A into 4R loads at the 50W/4R rating it has.
source: I'm just going by the formulas I found when googling for "electrical formula circle" and I'm assuming that's how you apply them. But I could also be sniffing glue.
