I believe your comparison doesn't make any sense. From what I've read:
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Considering your example, going from 25W to 100W doubles the power output twice and should provide a 6dB increase. That's a VERY noticeable increase. Going from 375W to 450W probably won't amount to more than 1 dB or so increase in output. Is a 1dB increase going to be very noticeable?
I'm not trying to pick a fight, and I'm sure you do hear it, but your example doesn't make much sense. I think folks should consider this before they dump their 6.0 of other amp for a 9.0.
+1 You need to take the denominator (i.e., the level of power you are starting with) into account.
However, as I've posted (and experienced) many times before, just translating an increase in power into db's can be very misleading. It's not always about pure volume, but rather, an uncompressed, open, clean, deep, quick tone. Many times, going from 'not quite enough power' to 'just enough or more than enough power' can result in a night and day experience (especially if you are starting at a modest wattage level in the 200-300 watt range).
Going from 300 to 450 watts, with a cabinet that can handle the extra power like a 410 or 212, can REALLY sound more open, full, and yes, a bit louder, which is a bonus. Once you get up to 500-600 watts, for most applications and tones, adding more power won't really make that much of a difference unless you are running multiple large cabs and playing VERY loud.
I see the primary application of a head like this running either one large 8ohm cab (like the fEarful 15/6's), or two large 410 8ohm cabs. For a single 4ohm cab or two smaller 8ohm cabs, if you dig the Genz tone, the 6.0 should be plenty for most. IMO!