The reason is that the impedance spectrum across the frequency range of the new speakers represents an easier load to your tube amplifier in addition to the fact that the true sensitivity is only 1dB difference, as per @atmasphere.
Sensitivity 86 v 90
I am considering an upgrade from speakers with a sensitivity rating of 90 db, 4 ohms, to larger speakers rated at 86db, 8 ohms nominal. Same brand, PMC. My tube integrated amp is 80-112 watts triode/ultralinear, and it’s fine for my 90 db speakers. Although, it is sometimes at around 4 o’clock on the volume control, approaching the max at 6. I am aware of the “amp power must double for each 3db increase in volume” rule of thumb, but really have no practical experience with this. I do like having 90 db efficiency, always assumed that meant a less powerful amp would suffice.
My question is, would the decreased efficiency be a concern?