My little Monoprice Pure Tube integrated puts out 10 watts per into 4 ohms, or about half that into 8 ohms.
I have a pair of Elac Unifi UB5 (original) rated at 85Db & 4 ohms, and a pair of Cambridge Audio Aeromax 2 rated at 90Db & 8 ohm?
Which is better for moderate volume & ease for the amp to drive?
I fully agree with the "try them both and use whichever one sounds best to you" approach. It's your room, your music, and your ears.
However, since this is actually a tube amp the power output difference between using 4 ohm vs 8 ohm speakers is not the same as it would be with a SS or Class D amp or a hybrid that uses a SS or Class D output section.
Those types typically do produce significantly less (roughly half in some cases?) power output with 8 ohm speakers than they do with 4 ohms. There will be a difference with a traditional tube amp that uses output transformers but, as I understand it, the power output difference is not nearly that extreme.
No doubt those with more technical expertise can elaborate further.
As for which speaker will be easier to drive, a lot depends on the impedance curves of the speakers. Impedance isn't static, it varies with frequency. Tubes work best with impedance curves that have less variation and don't dip to extremely low levels. Crossover design also plays a big role, the simpler the better.
And I'm sure it would be helpful to know the measured primary impedance of the output transformer. Until you know that it's just speculation as to whether 4 ohm or 8 ohm would be closer to an ideal match with the output tubes being used. The amp just has one set of outputs per channel, not separate taps for 4 ohms and 8 ohms.
I noticed that the manufacturer claims the output is simply 10w and makes no distinction between different speaker impedances. So, again, just use your ears and your speakers and decide for yourself.
While it's nice to have some headroom for peaks (dynamics), if you measure the average SPL and compare it with your speaker's specs (I know . . . not typically accurate and impedance curves are not considered) you'll likely find that you're using very little wattage under normal listening conditions, especially in an office environment.
I have big, vintage, Yamaha M-60 SS power amp which has power output meters. It's rated for 210 wpc at 4 ohms, 185 wpc at 6 ohms, and 160 wpc at 8 ohms. I don't know how accurate the meters are but I assume they're not too far off. In normal listening they bounce around between 0.75w and 1.5w using vintage 8 ohm speakers which typically claim efficiency in the 87db to 92 db range.
