The Aerial 5t is a 4 ohm speaker. BTW, why do you feel that you need a 4 ohm speaker? The impedance changes with most speakers!"
Ok, well, I don't know anything about impedance changing. Not that I needed another variable in the mix, but if it's there, it's there! One thing I did not clarify earlier on because I was still learning, was that I want to stay with in the amps Class A rating, and at 8 ohms, that is only 30w/ch. By going with a 4 ohm speaker (as in, only 4 ohm), the amp is now making (advertised) 60w/ch, fully class A.
So I guess in short, my answer is volume and headroom. How much of that I'll need depends on the speaker and likely the source, too. Though my listening levels will tend to be low, sometimes moderate, I can appreciate knowing that I have some flexibility there. 60w/ch still isn't a lot, most amps today make 100/ch.
One thing thats hard to pin, is what is the minimum wattage any speaker needs to sound full or at it's potential. That varies with speakers, and I don't know how to validate what power they need minimum. Maybe most book shelfs only need 10 watts to sound good? Others, more? And thats where sensitivity comes into play. Most book shelf speakers, or stand mounts, seem to be rated 85-86. Not helpful with low output amps. 95 would be a different story, etc. But I seem to be finding more and more 4 ohm speakers,not so much highly sensitive ones. I do know Klipsch makes some, though. So, that said, I'm also open to highly sensitive speakers, I just don't know at what sensitivity level they are on par with twice the volume, which would only be needed as a reference point so I could calculate other sensitivity figures, too.