Be careful with the term "full power", do we know what it is, I guess not!! If he told you 2.5 V, he must have based the amps rated output higher than I assumed. I assume that amp's rated output is just a little over 200 W, may be 220 W maximum, based on the datasheet. But hey, he sells them so that's reasonable for him to use a higher rating, such as at 1% THD instead of 0.1% that I typically prefer. By the way, Warren is a nice guy, a very experienced hobbyist himself but probably not too technically into the details, electrically speaking..
Take a look of the output vs THD graph on the datasheet:
At 1%, you do get quite a bit more than 200 W, but at 0.1%, it would actually be a little below 200 W.
That's why using input sensitivity is not as meaningful as using gain. With gain, you can calculate almost exactly why pre amp output voltage you need to drive your target power amp to the output level of your choice, based on your target acceptable distortion limit. With sensitivity, you often has to guess what the manufacturer's basis were when they published, or told you the specifications.
Regardless, this amp is an easy load only if you are using the balanced outputs of a preamp. For your Yamaha that only has unbalanced outputs, if you do need 200 W 8 ohm or more, then it isn't an easy load at all, because the Yamaha will struggle to even produce 1.6 V cleanly. However, if you don't listen loud, don't have speakers with low sensitivity, and don't sitting too far from the speakers, even 1 V will be good enough for you. So, it depends on your listening habit, speaker sensitivity, impedance, and distance a lot. There are less to consider, or worry, if the power amp's gain is higher, say 29 dB or more, but for the Purifi's 25 to 25.5 dB, you do have to consider the other factors I mentioned. It's a very transparent amp, so, time to enjoy the great clean sound, congrats to your purchase.
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